Chemical Engineering Journal 270 (2015) 168–175
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Advanced fabrication and oil absorption properties of
super-hydrophobic recycled cellulose aerogels
Jingduo Feng, Son T. Nguyen, Zeng Fan, Hai M. Duong ⇑
Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, EA-07-05, Singapore 117575, Singapore
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
For the first time, paper waste is
converted into a green cellulose
aerogel by an advanced and costeffective method.
The material has high oil absorption
capacities of 40–95 times of its
weight.
The material shows stable
superhydrophobicity over five
months.
Cellulose fiber concentration affects
significantly the oil absorption
capability of aerogel.
The maximum absorption capacity of
the aerogel is achieved at 50 °C.
a r t i c l e
i n f o
Article history:
Received 14 December 2014
Received in revised form 5 February 2015
Accepted 6 February 2015
Available online 14 February 2015
Keywords:
Cellulose aerogel
Paper waste
Temperature effect
Oil spill
Absorption
Kinetics
a b s t r a c t
A facile and cost-effective synthesis method of biocompatible cellulose aerogels using recycled cellulose
fibers of paper waste and Kymene crosslinker is successfully developed for the first time. After coated
with methyltrimethoxysilane (MTMS) via chemical vapor deposition, the recycled cellulose aerogels
yields very stable super-hydrophobicity for over five months and excellent oil absorption capacities of
up to 95 g g1 with the 0.25 wt.% cellulose aerogel. Effects of different cellulose fiber concentrations,
different ratios of the cellulose and Kymene cross-linker composition, ambient temperatures and pH
values of the environment on the oil absorption behavior of the cellulose aerogels are also quantified
comprehensively. The experimental results show the cellulose aerogels yield the maximum absorption
capacity at 50 °C. Compared to the pseudo first-order model, the pseudo second-order model is more
validated for the oil absorption kinetics study. The recycled cellulose aerogels are promising for replacing
earth-unfriendly polymer-based oil sorbents due to their high oil absorption capacities.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
With the growth of off-shore petroleum industry and the necessity of marine oil transportation, oil spills have become one of the
most important threats to the marine ecosystem. Currently, the
methods used for oil spill cleaning can be mainly categorized into
⇑ Corresponding author.
E-mail address: [email protected] (H.M. Duong).
http://dx.doi.org/10.1016/j.cej.2015.02.034
1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
three groups: chemical methods, bioremediation and physical
methods [1,2]. Chemical methods including dispersion, in-situ
burning and solidification [3] are costly, not environment-friendly.
Bioremediation [4,5] using microorganisms to degrade hydrocarbon in the oil, is effective yet time-consuming, and its effectiveness
is affected dramatically by temperature, oxygen present and
organic species. The physical methods use booms and skimmers
to corral the oil [6–8]. However, their efficiency for removing oil
remains quite limited.
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J. Feng et al. / Chemical Engineering Journal 270 (2015) 168–175
In recent years, physical absorption has attracted much attention for the oil spill cleanup issues, owing to its low cost, facile
operation and low energy consumption [9,10]. There are basically
three categories of sorbents such as natural organic sorbents, inorganic sorbents and synthetic organic sorbents [11–14]. Their effectiveness for oil absorption is commonly evaluated by the mass of
oil up taken by per unit mass of the absorbent at its swelling equilibrium [10]. The natural organic sorbents such as rice husk, coconut husk and sugar cane bagasse usually possess the oil absorption
capacity of 2–4 g g1 [2,15]. To improve their oil absorption capacity, pyrolysis is used to enhance the porosity of the materials
and make them more oleophilic. As a result, the pyrolyzed materials have higher oil absorption ability of 7–10 g g1 [15–18]. Inorganic sorbents such as vermiculite, fly ash and manganese oxide
nanofiber membranes have the larger oil absorption capacity of
less than 20 g g1 [19,20]. Besides the low oil absorption capacity,
these natural organic and inorganic sorbents also suffer the problems of absorbing both oil and water during the absorption process
[21]. Such poor hydrophobicity significantly reduces the absorption efficiency in their industrial applications.
Several efforts [14,19–31] have been made to develop synthetic
organic sorbents with low cost, high oil absorption capacity and
good oil–water selectiveness. Commercial polypropylene (PP) fiber
mats with excellent hydrophobicity and the oil absorption capacities of approx. 15 g g1 were developed, but their degradation
remained a major environmental challenge [14]. To overcome this
problem, cellulose aerogels [22–26] composed of biodegradable
cellulose fibers and inherited the high porosity and large surface
area of aerogels [27,28], were developed as a novel type of the synthetic organic sorbents having the high absorption capacities of
over 20 g g1 [29]. The combination of the advanced aerogel properties with the biocompatible cellulose fibers, which are abundant
worldwide via woods, plants, algae and animals [30,31], make the
cellulose aerogels a very promising candidate for oil absorption
applications. The development of recycled cellulose aerogels having low density and high porosity from paper waste were developed through an alkaline/urea method [32,33]. After subsequent
hydrophobic coating, the recycled cellulose aerogels showed the
very high absorption capacities of 24.4 g g1 for crude oils. These
developed cellulose aerogels can reduce further the fabrication
cost due to the low cost of the paper waste.
In this paper, we report an advanced and cost-effective method
to synthesize the recycled cellulose aerogels from paper waste by
using Kymene as a cross-linker, instead of sodium hydroxide and
urea [24,25,34]. This novel method can reduce significantly the
toxicity of raw materials, and the entire synthesis duration from
7 days of previous methods [32,33] down to 2 days. After freezedried and coated with methyltrimethoxysilane (MTMS) via
chemical vapor deposition, the recycled cellulose aerogels exhibit
ultra-flexibility, highly porosity, super-hydrophobicity and outstanding oil absorption capability.
2. Experiments
2.1. Materials
Recycled cellulose fibers and Kymene 557H are sponsored by
Insul-Dek Engineering Pte. Ltd. (Singapore) and Ashland
(Taiwan), respectively. All the solutions are made with deionized
(DI) water. Motor oils of 5w40 and 5w50 are purchased from
Carlube and Singer machine oil is purchased from the commercial market. Analytical grade MTMS, sodium chloride (NaCl),
hydrochloric acid (HCl), sodium hydroxide (NaOH) and Sudan
Red G are purchased from Aldrich Sigma. All these chemicals
are used as received.
2.2. Fabrication of the recycled cellulose aerogels
The recycled cellulose fibers from paper waste (0.075–0.3 g)
and Kymene (5–20 ll) are first dispersed in 30 ml DI water by
sonicating the mixtures for 10 min. The suspensions are then
placed in a refrigerator at 18 °C for more than 24 h to allow the
gelation. The cellulose aerogels are obtained by freeze drying the
obtained gels at 98 °C for 2 days using a Scan Vac CoolSafe 95–
15 Pro freeze dryer (Denmark). Thereafter, the cellulose aerogels
are further cured at 120 °C for another 3 h to cross-link completely
the Kymene molecules. Various cellulose aerogels with different
compositions of the cellulose fibers and Kymene are synthesized
and summarized in Table 1. Porosities of the aerogel samples are
calculated using the following formula.
q
Ø ¼ 100 1 a
ð1Þ
qc
where qa is the density of the aerogel and qc is the density of crystalline cellulose (1.5 g/cm3) [35].
2.3. Development of the super-hydrophobic recycled cellulose aerogels
As the recycled cellulose aerogels developed in Part 2.2 are
hydrophilic, the as-prepared cellulose aerogels are coated with
MTMS on their highly porous networks [29,35,36] to form the
super-hydrophobic cellulose aerogels for the oil absorption. The
cellulose aerogel sample and an open glass vial containing MTMS
are placed in a big container. The container is then capped and
heated at 70 °C for 3 h for the silanation reaction [33]. After the
aerogel structure is coated completely, excessive MTMS is removed
by placing the aerogel sample in a vacuum oven until the pressure
goes down below 0.03 mbar.
2.4. Characterization
Surface morphologies of the developed super-hydrophobic cellulose aerogels (sample A–E) are investigated using a scanning
electron microscopy (SEM, JSM-600LV of Japan).
Water contact angle measurements are carried out on a VCA
Optima goniometer (AST Products Inc., USA) to evaluate the water
repellency of the MTMS-coated aerogels. During the test, water
drops of 0.5 ll are controlled by the syringe system of the tester,
and dispensed drop by drop onto the surface of the samples. The
contact angle values are calculated using the contact angle meter
software on basis of the droplet shape in the image. For each sample, measurements are repeated at several different positions, and
its contact angle is finally determined by averaging these contact
angle values from various measurements.
To investigate the oil absorption capability of the super-hydrophobic recycled cellulose aerogels, oil absorption test is conducted according to a modified ASTM F726-06 standard. Dry aerogels
having dimensions of 45 mm (diameter) 11 mm (thickness) are
first weighed, and then immersed in 800 ml motor oil (5w40) for
2 h to ensure a swelling equilibrium. Thereafter, the gels are lifted
up from the oil container by a stainless steel mesh basket, drained
Table 1
Chemical compositions of various recycled cellulose aerogels.
Sample label
Cellulose fibers (wt.%)
Kymene (ll)
Porosity (%)
Sample
Sample
Sample
Sample
Sample
Sample
0.25
0.50
0.75
1.00
0.60
0.60
5
5
5
5
5
20
99.4
98.9
98.1
97.2
n.a.
n.a.
A
B
C
D
E
F
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J. Feng et al. / Chemical Engineering Journal 270 (2015) 168–175
in air for 30 s and weighed again. The oil absorption capacity of the
aerogels is calculated using the formula as below:
Qt ¼
mw md
md
ð2Þ
where Qt is the oil absorption capacity of the aerogel at time t. md
(g) and mw (g) are the aerogel weights before and after the oil
absorption, respectively.
In order to further study the oil absorption kinetics of these cellulose aerogels, the immersed aerogels are lifted up from the oil,
drained and weighted at several time intervals (0, 2, 5, 7, 10, 15,
20, 30 and 60 s). The motor oil (5w50) and the Singer machine
oil having different viscosities of 0.14 and 0.026 Pa s, respectively
are also used for the absorption kinetics tests to investigate effects
of the oil viscosity on the absorption kinetics.
3. Results and discussions
3.1. Morphologies of the super-hydrophobic recycled cellulose aerogels
Fig. 1 shows the photographs and SEM images of the developed
recycled cellulose aerogels. The aerogel sample in Fig. 1a has a
dimension of 45 mm (diameter) 11 mm (thickness) and same
shape of its reaction container. As reported previously [33,34],
the recycled cellulose aerogels are formed via hydrogen bonding
between the self-assembled cellulose fibers. However, in this work,
Kymene molecules can diffuse and react with the cellulose fiber
surface to form the hydrogen bonding, and also crosslinks with
the surrounding Kymene molecules. The utilization of Kymene as
a cross linker combines the reinforcement and protection mechanisms [37] during the gelation process, which thus ensure the resultant aerogels with a robust structure and good flexibility. As can be
seen in Fig. 1b, the large-scale cellulose aerogel is easily bent or
rolled without damaging its shape. However, when the cellulose
concentration increases, their flexibility tends to be slightly
decreased, as a result of the raised difficulty in sliding and bending
of the cellulose fibers.
In contrast to the mesopores (2–50 nm) of the aerogels formed
by the cellulose nanofibers, highly porous structures of the cellulose aerogels with macropores (>50 nm) can be clearly observed
in SEM images of Fig. 1c–f. Their macropores are possibly caused
by the larger size of the recycled cellulose fibers, obtained from
the paper waste [33]. Fig. 1c and d shows the morphologies of
the cellulose aerogels with cellulose concentrations of 0.25 and
1.00 wt.% respectively. The aerogel with the higher cellulose concentration (1.0 wt.%) has a more compacted network and lower
porosity. However, an increase of Kymene from 5 to 20 ll in a
30 ml reaction mixture does not significantly impact the aerogel
structures as shown in Fig. 1e and f.
In order to investigate the super-hydrophobicity of the developed cellulose aerogels, the water contact angles are measured
on both the external and internal surfaces of the MTMS-coated cellulose aerogels. As shown in Fig. 2a and b, the large contact angles
of 153.5° and 150.8° are obtained respectively, thus proving that
the hydrophobic coating is successfully covered the whole aerogel
networks. The effects of reagent ratios on the super-hydrophobic
properties of the aerogels with different compositions are also presented in Fig. 3. Interestingly, all cellulose aerogels exhibit similar
Fig. 1. (a) Super-hydrophobic recycled cellulose aerogel, (b) flexibility of the large-scale cellulose aerogel (38 cm 38 cm 1 cm) containing 0.60 wt.% of the cellulose fibers,
SEM images of the cellulose aerogels with different ratios of cellulose fibers (wt.%) and Kymene (ll): (c) 0.25:5, (d) 1.00:5, (e) 0.60:5 and (f) 0.60:20.
Fig. 2. Water contact angles on (a) the external surface and (b) the cross section of the super-hydrophobic recycled cellulose aerogel.
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J. Feng et al. / Chemical Engineering Journal 270 (2015) 168–175
confirm the uniform MTMS coating, the excellent hydrophobicity
stability of the MTMS-coated recycled cellulose aerogels in this
work.
3.2. Oil absorption capabilities of the recycled cellulose aerogels
Fig. 3. Effects of exposure time on the water contact angles: (a) of the cellulose
aerogels with different ratios of the cellulose fibers and Kymene, and (b) on the
external surface and the cross section of the same aerogel.
water contact angles of approx. 150°, regardless of their cellulose
concentrations or Kymene amount in Fig. 3a. It is well-known that
the water contact angles strongly depend on the functional groups
on the aerogel surfaces. Therefore, in this case, such a small variation may likely be a consequence of the identical functional groups
(ASiAOACH3A) induced by the MTMS coating [33].
To examine the hydrophobic stability of the cellulose aerogels,
they are then exposed in normal ambient atmosphere for five
months. Their water contact angles of 145–155° over the period
are examined as can be seen in Fig. 3a and b. Apparently, their
water contact angles on the external surface and the cross section
do not show any obvious change with time. These results further
Table 2
Viscosities of the tested oils at different temperatures.
Viscosities (Pa s)
25 °C
50 °C
70 °C
5w40 motor oil
5w50 motor oil
Singer machine oil
0.140
0.160
0.026
n.a.
0.054
0.009
n.a.
0.029
0.006
A 5w40 motor oil is used to investigate the oil absorption capabilities of the recycled cellulose aerogels in Table 2. When the
Kymene amount is kept to be 5 ll and increasing the cellulose concentration from 0.25 to 1.00 wt.%, the measured absorption capacities of the aerogels A–E in Table 2 are 95, 73, 58 and 49 g g1,
respectively at 25 °C. The maximum absorption capacity of
95 g g1 is achieved with the 0.25 wt.% cellulose aerogel due to
its lowest density (7 103 g/cm3) and highest porosity (99.4%).
The absorption capacities of all the MTMS-coated cellulose
aerogels are one order greater than those of the nature sorbents,
2–4 times greater than those of the commercial polypropylene sorbents [13–15], and also 5 times higher than those of the recycled
cellulose aerogels (20 g g1) reported by the previous works using
the sodium hydroxide/urea method [32,33]. The significant
enhancement of the absorption capacity may be largely ascribed
to the reduced densities and increased porosities of the cellulose
aerogels. Without using urea and hydroxide residuals, the densities
of the cellulose aerogels cross-linked by the Kymene are consequently lowered with their weights.
3.3. Effects of the cellulose fiber concentrations, oil types and
temperatures on the oil absorption kinetics of the recycled cellulose
aerogels
The absorption kinetics of the 5w50 motor oil and Singer
machine oil onto the recycled cellulose aerogels are investigated
and summarized in Table 3. Although sample A with cellulose fiber
concentration of 0.25 wt.% (Table 1) has the highest oil absorption
capacity, it possesses a weak mechanical strength and is easily to
disintegrate after absorbing oil. Therefore, three different aerogel
samples marked as B, C and D (Table 1) having cellulose fiber concentrations of 0.50, 0.75 and 1.00 wt.% are prepared for the oil
absorption kinetics tests at 25 °C. As shown in Fig. 4, the sorption
capacity of each oil on the cellulose aerogels is plotted as a function
of absorption time. The sorption rate is fast at the first 10 s and the
absorption reaches the equilibrium state at 30 s for both of the two
oils.
Temperature is also the major factor affecting the viscosity and
the diffusion rate of the oils into the porous aerogel structures.
Therefore, the absorption behavior of the different oils on each aerogel is examined at three different temperatures of 25, 50 and 70 °C.
As shown in Fig. 5 and Table 3, the maximum oil absorption capacity
Table 3
Summary of the maximum oil absorption capacities and the absorption rate constants at different temperatures of the cellulose aerogels having various cellulose fiber
concentrations using the pseudo-first and -second order models.
⁄
Cellulose concentration (wt.%)
0.50
Temperature (°C)
25
50
70
25
50
70
25
50
70
5w50 motor oil
Maximum absorption capacity, Qm (g/g)
Pseudo-first order
R2⁄
k1
Pseudo-second order
R2⁄
k2
62.6
0.9858
0.2929
0.9955
0.0056
64.9
0.9701
0.3235
0.9962
0.0082
59.2
0.9717
0.3552
0.998
0.0098
48.1
0.977
0.2539
0.9946
0.0075
53.8
0.9544
0.3187
0.9978
0.0098
46.3
0.9658
0.3671
0.9964
0.0153
45.9
0.9857
0.2176
0.9956
0.0048
46.5
0.9926
0.2866
0.9949
0.0066
46.2
0.9437
0.3574
0.998
0.0133
Singer machine oil
Maximum absorption capacity, Qm (g/g)
Pseudo-first order
R2⁄
k1
Pseudo-second order
R2⁄
k2
59.3
0.9825
0.2965
0.9942
0.0075
61.0
0.978
0.3110
0.9992
0.0101
58.2
0.9803
0.3514
0.9987
0.0123
46.1
0.9808
0.2734
0.9964
0.0083
48.8
0.9557
0.3063
0.995
0.0113
47.6
0.9935
0.3432
0.9989
0.0146
40.4
0.9782
0.2782
0.9929
0.0070
43.1
0.9704
0.3768
0.9936
0.0092
42.4
0.9871
0.4252
0.9959
0.0161
R2 is the correlation coefficient.
0.75
1.00
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J. Feng et al. / Chemical Engineering Journal 270 (2015) 168–175
increases when increasing the temperature from 25 to 50 °C, but
then decreases with the temperature change from 50 to 70 °C. This
trend occurs to the absorption behavior of all the oils on the 0.50,
0.75 and 1.00 wt.% cellulose aerogels. It may be explained that the
temperature increase can reduce the oil viscosity in Table 2, which
facilitates oil penetration into the porous aerogel networks. However, the low viscosity of the oils has a negative effect on their anchoring to the pore walls, which reduces the amounts of the oil retained
in the porous absorbents. Comparing the maximum oil absorption
capacity to the tested temperatures, it can be concluded that 50 °C
is the optimum temperature for the oil absorption performance of
the recycled cellulose aerogels.
Beside the temperature effects, the porous structure of the cellulose aerogels also significantly affects their oil absorbency. Fig. 5
and Table 3 show the oil absorption capacity of the aerogels
reduces when the initial cellulose concentration increases from
0.50 to 1.00 wt.%. It can be explained based on the porosity of
the cellulose aerogels. Table 1 shows the aerogel porosity reduces
from 98.9% to 97.2% when the cellulose concentration increases
from 0.50 to 1.00 wt.%. As the aerogel porosity is lower, there is less
space in the aerogel network for oil occupation and therefore, the
oil absorbency is less.
Various kinetics models have been suggested for absorption
analysis [38–40]. The pseudo-first order and pseudo-second order
models [11,41–44] are commonly used for the oil absorption. In
this work, both of the models are used to fit the experimental
absorption kinetics data. After integration, the pseudo-first order
equation can be obtained [41]:
ln
Fig. 4. Absorption kinetics of (a) the 5w50 motor oil and (b) the Singer machine oil
onto the recycled cellulose aerogels with various cellulose fiber concentrations of
0.50, 0.75 and 1.00 wt.% at 25 °C.
Qm
¼ k1 t
Qm Qt
where Qm (g/g) is the maximum oil absorbency, Qt is the oil
absorbency at time t, and k1 is the absorption rate constant determined from the slope of ln[Qm/(Qm Qt)] versus t plot.
And the pseudo-second order equation can be converted into a
linear form [40]:
t
1
1
¼
tþ
Qt Qm
k2 Q 2m
Fig. 5. Maximum absorption capacities, Qm of (a) the 5w50 motor oil and (b) the
Singer machine oil onto the recycled cellulose aerogels with various cellulose fiber
concentrations of 0.50, 0.75 and 1.00 wt.% at 25, 50 and 70 °C.
ð3Þ
ð4Þ
By plotting (t/Qt) versus t, the absorption rate constant k2 can be
determined. The pseudo first-order model can be used in many
absorption cases, such as systems close to equilibrium, systems
with time-independent solute concentration or linear equilibrium
absorption isotherm [40] while the pseudo second-order model
is used to describe the sorption process controlled by chemisorption [39,42,45–48].
For the absorption of the 5w50 motor oil and Singer machine oil
on the 0.50 wt.% cellulose aerogel at 25 °C, respectively, Fig. 6a and
b shows the plots of ln[Qm/(Qm Qt)] versus time t using the pseudo-first order model while Fig. 6c and d displays the plots of (t/Qt)
against t using the pseudo-second order model. Fig. 7a and b shows
the experimental absorption kinetics data the two fitting model
curves for the absorption of the two oils on the 0.50 wt.% aerogel
samples. From the plots of Fig. 6, the sorption rate constants k1,
k2 and the correlation coefficient R2 are calculated and presented
in Table 3. It can be seen that the correlation coefficient values of
the pseudo second-order model are higher than those of the pseudo first-order model for both tested oils. So the pseudo secondorder model can predict better the oil absorption behavior in this
work. Most of the absorption rate constants k1 and k2 for the
5w50 motor oil are smaller than those for the Singer oil. This
means the oil absorption of the Singer oil occurs faster due to its
lower viscosity.
Activation energy, Ea is an important parameter in a thermodynamic study [49]. In absorption, the energy must be overcome by
J. Feng et al. / Chemical Engineering Journal 270 (2015) 168–175
173
Fig. 6. Pseudo-first order absorption linear fitting of (a) the 5w50 motor oil and (b) the Singer machine oil and Pseudo-second order absorption linear fitting of (c) the 5w50
motor oil and (d) the Singer machine oil onto the aerogel having 0.50 wt.% of the cellulose fibers at 25 °C.
lnk ¼ lnA Ea
RT
ð5Þ
where A is the pre-exponential factor and R is the gas constant
(8.314 J/mol K). By plotting ln[k] against 1/T, Ea can be calculated
from the slope.
Plots of ln[k1] and ln[k2] versus 1/T are presented in Fig. 8a–d
and the activation energy values are presented in Table 4. It can
be observed that the activation energy values of the pseudo second-order model are higher than those of the pseudo first-order
model. This is because the pseudo second-order model is used
for the absorption process controlled by chemisorption, which
involves higher forces than in physic-sorption [39]. Compared to
the 5w50 motor oil, the Singer machine oil has the lower activation
energy values which make the oil absorption on the cellulose aerogels more effective.
3.4. Effects of environmental conditions on the oil absorption
performance of the recycled cellulose aerogels
Fig. 7. Experimental data fitted with the pseudo-first and -second order models for
the absorption kinetics of (a) the 5w50 motor oil and (b) the Singer machine oil
onto the aerogel having 0.50 wt.% of the cellulose fibers at 25 °C.
an absorbate to interact with functional groups on the sorbent surface. The activation energy can be determined from the change of
the absorption rate constant, k with temperature, T (K) using the
Arrhenius equation [38,49]:
To evaluate the practical oil absorption performance of the
recycled cellulose aerogels in the sea, a 3.5% NaCl solution is prepared to imitate the artificial seawater. Fig. 9 depicts the absorption process in the first several minutes. It is visible that the
cellulose aerogel is loaded on the top of the mixture and quickly
absorbs the motor oil within 7 min. Also the pH effects on oil
absorption capacities of the cellulose aerogels are investigated by
using the artificial seawater with different pH values prepared
from HCl or NaOH. The oil absorption capacities of the 0.50 wt.%
cellulose aerogels under the pH = 3, 5, 7 and 9 environment are
measured to be 63.00, 62.85, 63.06 and 62.98 g g1, respectively,
The absorption results indicates a pH-insensitive behavior of the
aerogels during the oil absorption tests. This may be possibly
because the oil capacities of the aerogels are mostly contributed
by their porosities and tested oil viscosities, while both of them
are independent on the environmental pH values.
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J. Feng et al. / Chemical Engineering Journal 270 (2015) 168–175
Fig. 8. Plots of ln(k1) and ln(k2) against reciprocal temperature for the absorption of (a and b) the 5w50 motor oil and (c and d) the Singer machine oil, respectively onto the
cellulose aerogels with various cellulose fiber concentrations of 0.50, 0.75 and 1.00 wt.%.
Table 4
Activation energies of the absorption of the cellulose aerogels having various cellulose concentrations on the oils using the pseudo-first and -second order models.
Activation energies, (J/mol)
Initial cellulose concentration (%)
0.50
0.75
1.00
5w50 motor oil
Pseudo-first order
Pseudo-second order
3609.6
10682.7
6984.6
13120.3
9331.6
18612.6
Singer machine oil
Pseudo-first order
Pseudo-second order
3090.5
9354.9
4248.0
10610.3
8130.2
15240.4
Fig. 9. Oil absorption process of the recycled cellulose aerogel having 5.0 wt.% of the cellulose fibers in the artificial seawater (3.5% NaCl and pH = 7) mixed with the 5w40
motor oil and dyed with Sudan Red G before testing.
4. Conclusions
In conclusion, an advanced and cost-effective method of the
recycled cellulose aerogels from paper waste is successfully devel-
oped. After coated with MTMS, the developed cellulose aerogels
exhibit excellent oil absorption capacities and very stable superhydrophobicity for over five months. It is found that the initial cellulose fiber concentration significantly affects the oil absorption
J. Feng et al. / Chemical Engineering Journal 270 (2015) 168–175
capability of the developed cellulose aerogels. The 0.25 wt.% cellulose aerogel yields the maximum absorption capacity of 95 g g1
with the 5w40 motor oil.
The maximum absorption capacity of the cellulose aerogels can
be achieved at 50 °C, regardless the pH values of the seawater/oil
suspensions and decreases with the increase of the cellulose fiber
concentration. The pseudo-first and -second order kinetics models
are applied to describe the oil absorption behavior of the recycled
cellulose aerogels for the first time. The pseudo second-order model is more validated for the oil absorption kinetics study of the
aerogels. The experimental results of this work demonstrate our
super-hydrophobic recycled cellulose aerogels could be one of
the very promising sorbents for oil spill cleaning.
Acknowledgements
The authors would like to thank Singapore National
Environment Agency – Environment Technology Research Program
(7th RFP) (R-265-000-450-490) for the financial support for the
project. We are grateful of Insul-Dek Engineering Pte. Ltd.
(Singapore) and Ashland (Taiwan) for giving us recycled cellulose
fibers and Kymene, respectively. We appreciate Dr. Lin Yuan for
help in measuring oil viscosity.
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