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Article
Synthesis, Characterization, and Utilization of a Lignin-Based
Adsorbent for Effective Removal of Azo Dye from Aqueous Solution
Xianzhi Meng,* Brent Scheidemantle, Mi Li, Yun-yan Wang, Xianhui Zhao, Miguel Toro-Gonzaĺ ez,
Priyanka Singh, Yunqiao Pu, Charles E. Wyman, Soydan Ozcan, Charles M. Cai,
and Arthur J. Ragauskas*
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ABSTRACT: How to effectively remove toxic dyes from the
industrial wastewater using a green low-cost lignocellulose-based
adsorbent, such as lignin, has become a topic of great interest but
remains quite challenging. In this study, cosolvent-enhanced
lignocellulosic fractionation (CELF) pretreatment and Mannich
reaction were combined to generate an aminated CELF lignin
which is subsequently applied for removal of methylene blue and
direct blue (DB) 1 dye from aqueous solution. 31P NMR was used
to track the degree of amination, and an orthogonal design was
applied to determine the relationship between the extent of
amination and reaction parameters. The physicochemical,
morphological, and thermal properties of the aminated CELF
lignin were characterized to confirm the successful grafting of
diethylenetriamine onto the lignin. The aminated CELF lignin proved to be an effective azo dye-adsorbent, demonstrating
considerably enhanced dye decolorization, especially toward DB 1 dye (>90%). It had a maximum adsorption capacity of DB 1 dye
of 502.7 mg/g, and the kinetic study suggested the adsorption process conformed to a pseudo-second-order kinetic model. The
isotherm results also showed that the modified lignin-based adsorbent exhibited monolayer adsorption. The adsorbent properties
were mainly attributed to the incorporated amine functionalities as well as the increased specific surface area of the aminated CELF
lignin.
industrial wastewater has become a topic of great interest.
Adsorption is considered as an alternative method to the
traditional combination of chemical and biological processes
for the removal of dyes from aqueous solutions.4,5 Dye
adsorption by various adsorbents is considered to occur
primarily via π interaction, hydrogen bonding, and electrostatic
interactions. Several organic and inorganic materials including
zeolite,2,6 lignocellulosic substrate,7,8 activated carbon,9 graphite,10 and graphene oxide have been all tested and shown to
have different adsorption activities toward organic dyes in
wastewater.11,12 Some of these adsorbents have a rather high
dye-removal efficiency; however, low-cost renewable green
bioadsorbents are still rare for this field of application, and
further studies are urgently required.13
The biorefinery concept has received considerable attention
in the last decade because of advances in biotechnology and
INTRODUCTION
The demand for clean water is likely to increase driven by the
rapid urbanization, expanding industrial activities, energy
generation, and water pollution. Because of the limited fresh
water resources on earth, this demand should be addressed by
developing promising water purification techniques. The
presence of organic dyes in industrial wastewater could cause
some serious environmental concerns because of their poor
biodegradability and toxicity to the exposed plants, living
organisms, and even human being. As a result, these toxic dyes
should be removed from the wastewater as much as possible
before being discharged to land or water sources in an
environmentally friendly manner.1 The textile industry
consumes organic dyes, which represent 60% of the world’s
dye consumption, and it was reported that 10−25% of watersoluble dyes is lost during the dyeing process, and 2−20% of
dyes is released as the effluent into the water system after the
dyeing process.2 Azo dyes are known as dyes containing −N
N− groups, representing 60−70% of commercially available
dyes in the world.3 They are extensively used in the textile
industry and become part of the textile effluents. Therefore,
how to cost-effectively remove these toxic azo dyes from the
■
© XXXX American Chemical Society
Received: November 2, 2019
Accepted: January 23, 2020
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obtained final aminated lignin was thoroughly characterized by
various analytical techniques including Fourier-transform
infrared spectroscopy (FTIR), nuclear magnetic resonance
spectroscopy (NMR), scanning electron microscopy (SEM),
thermal gravimetric analysis (TGA), and Brunauer−Emmett−
Teller (BET) surface area analysis. Finally, the performance of
the obtained aminated CELF lignin as a biorenewable
adsorbent for the removal of methylene blue (MB) and direct
blue (DB) dyes was evaluated and compared to other reported
adsorbents from literature. The combinatorial process takes
advantage of the selectivity of the Mannich chemistry and the
unique versatile functionality of CELF lignin such as low ether
linkages and high phenolic OH content. It is fully expected that
this study will provide a baseline for future studies to
synthesize a renewable lignin-based dye adsorbent in advanced
wastewater treatment systems.
genetic engineering, offering a renewable and sustainable
alternative to the production of common petrochemicals.14
Abundant lignocellulosic biomass is a second-generation
nonfood feedstock that, when used by future biorefineries,
has the potential to significantly offset the carbon footprint of
the traditional refining.15 As one of the most important
renewable fractions found in biomass, lignin is still significantly
underutilized in the current biorefinery industries, which has
mainly focused on transforming biomass carbohydrates to
liquid fuels.16 It is anticipated that with the growing demand of
biomass for production of fuels, the production of lignin would
also substantially increase, potentially serving as a versatile
platform for the production of biopolymers and renewable
high-performance materials. The Renewable Fuel Standard
(RFS) established in 2005 and further expanded in 2007 by
the Energy Independence and Security Act (EISA) aims to
ascend to 36 billion gallons of renewable fuel in 2022.
Assuming a yield of 335 L per dry Mg of biomass, 223 million
Mg of biomass will be used annually, producing about 62
million Mg of lignin.17 To avoid using lignin as a low-grade
boiler fuel, new thermal and chemical processes are needed to
generate value-added products from lignin.
A lignin macromolecule contains various amounts of
functional groups including carbonyl, methoxy, carboxyl, and
hydroxyl groups, which offers promising opportunities to take
advantage of its versatile functionality for multiple applications.18 Without any further chemical treatments, lignin could
be directly incorporated into a polymeric matrix to be served as
an antioxidant,19 a flame retardant,20 dye adsorbent,21,22 and a
UV stabilizer.23 Given the diversity of lignin, variability in
performance as a functionalized polymer is expected to depend
on the plant sources, lignin isolation methods, and
physicochemical structures of lignin. Thus, chemical modifications of lignin to improve its valorization have attracted
growing attention.24,25 Lignin amination refers to a process
that introduces an amine group into the lignin structure. One
of the bases of lignin amination is the Mannich reaction that
refers to the reaction between the lignin and amine in the
presence of formaldehyde. The obtained aminated lignin has
properties that make it ideal for use in several applications,
including surfactants,26 dispersants,27 heavy metal adsorbents,28 and asphalt emulsifiers.29,30 Because of its aromatic/
phenolic nature and cationic side chain, the aminated lignin
could have great potential as a low-cost bioadsorbent for the
removal of organic dyes especially azo dyes that are typically
anionic in charge in wastewater.
Here, we used a cosolvent enhanced lignocellulosic
fractionation (CELF) method as a highly effective lignin-first
pretreatment approach that is capable of extracting highly pure
technical-grade lignin from corn stover. CELF applies aqueous
mixture of tetrahydrofuran (THF) and dilute acid to greatly
enhance the fractionation of lignin, hemicellulose, and cellulose
fractions in biomass while promoting lignin fragmentation by
limiting certain lignin condensation reactions typically suffered
at high reaction severities. The obtained CELF lignin is
depolymerized, containing lower aryl ether linkages and higher
phenolic hydroxyl groups than the typical native milled wood
lignin, cellulolytic enzyme lignin, or kraft lignin, which favors
the subsequent amination process.31,32 The isolated CELF
lignin was then aminated by diethylenetriamine (DETA) in the
presence of formaldehyde under acid conditions via the
Mannich reaction. An orthogonal array system was also applied
to test the optimal conditions for the Mannich reaction. The
RESULTS AND DISCUSSION
Synthesis and Characterization of Adsorbents.
Orthogonal Experiments. Systematic experimental designs
such as response surface methodology and orthogonal arrays
are widely used to obtain the optimal response.33,34 An
orthogonal experiment design (L16, 54) including five factors
(A: temperature, B: time, C: amine content, D: formaldehyde
content, and E: acetic acid content) at four different levels was
first applied to determine the optimal Mannich reaction
conditions. The Mannich reaction can only occur between a
high electron density carbon and an immonium ion formed
from formaldehyde and an amine, thus the amine groups are
expected to be introduced only at the ortho or para position of
a phenolic hydroxyl group, converting H or G types of free
phenolic hydroxyl group (or both) to C5 substituted hydroxyl
groups (Figure 1).35 This provides a unique opportunity to
■
Figure 1. Mannich reaction between the phenolic G/S/H lignin and
DETA, leading to the formation of phenolic C5 substituted lignin
units.
determine the extent of lignin amination by tracking the
content of free H and G phenolic hydroxyl groups.
Quantitative 31P NMR technique was used to track the
content of free H and G phenolic hydroxyl groups, aiming to
assess the extent of lignin amination. Table 1 shows the
conversion of H and G types of free phenolic hydroxyl for each
experiment of the designed orthogonal array. As indicated by
Table 1, the effect of each experiment parameter on the extent
of lignin amination increases in this order: acetic acid content
(E) < reaction time (B) < DETA content (C) < formaldehyde
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Table 1. L16 (P5)L4 Orthogonal Experiment Design of the
Mannich Reaction between CELF Lignin and DETA under
Acid Conditionsa
experimentb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
K1c
K2
K3
K4
Rd
best quality
level
optimal
combination
temp.
(A)
time
(B)
DETA
(C)
formal.
(D)
acid
(E)
A1
A1
A1
A1
A2
A2
A2
A2
A3
A3
A3
A3
A4
A4
A4
A4
49.7
64.6
84.0
70.4
34.3
A3
B1
B2
B3
B4
B1
B2
B3
B4
B1
B2
B3
B4
B1
B2
B3
B4
58.5
69.1
70.0
71.1
11.6
B4
C1
C2
C3
C4
C2
C1
C4
C3
C3
C4
C1
C2
C4
C3
C2
C1
76.3
64.9
65.3
62.2
13.1
C1
D1
D2
D3
D4
D3
D4
D1
D2
D4
D3
D2
D1
D2
D1
D4
D3
60.9
61.7
71.3
74.8
13.9
D4
E1
E2
E3
E4
E4
E3
E2
E1
E2
E1
E4
E3
E3
E4
E1
E2
67.0
70.3
64.9
66.4
2.1
E2
(G + H)
phenolic
conversion
(%)
43.8
46.9
52.5
55.6
57.1
81.0
59.4
61.1
84.3
85.0
89.8
77.1
48.9
63.4
78.3
90.8
Figure 2. FTIR spectra of the corn stover CELF lignin and aminated
CELF lignin.
stretching vibrations of −CH3 and −CH2− (2844 cm−1) are
observed in both lignin samples. Nonetheless, there exist
obvious differences between the lignin and aminated lignin
samples. For example, the intensity of FTIR peaks associated
with the aromatic C−H vibrations including 1603 and 1512
cm−1 from the aminated lignin is significantly lower than that
from the original CELF lignin. This is because the Mannich
reaction mainly occurred in the aromatic region of lignin.37 In
addition, the intensity of FTIR peaks associated with G units
including 1267 cm−1 (C−O stretch), 1113 cm−1 (deformation
vibrations of C−H), and 1030 cm−1 (aromatic C−H in-plane
deformation) and H units such as 1164 cm−1 (C−O stretch) is
decreased after amination reaction.38 On the other hand, the
intensity of syringyl C−O stretch (1325 cm−1) remains
relatively strong after Mannich reaction.38 This is because
the amine group could be only introduced at the C3 or C5
position of H lignin and the C5 position of the G units. Finally,
a strong peak around 1650 cm−1 representing the N−H
bending vibrations in the amine structure (NH2) appeared in
the aminated lignin, validating the successful addition of the
amine.39 The peak of carbonyl group around 1680 cm−1 also
disappears after Mannich reaction possibly because of the
reaction between CO and primary amines to form imine
derivatives known as Schiff bases.40
2D HSQC Analysis. Two-dimensional HSQC NMR has
been comprehensively used in lignin characterization because
of its versatility in offering structural insight into the lignin
subunits and interlinkages.41 The reaction mechanism of
Mannich reaction and the chemical structural transformation
of CELF lignin during the amination reaction was further
characterized by 2D HSQC NMR in this study. As shown in
Figure 3, the CELF lignin possesses typical structural aromatic
patterns of corn stover lignin. Peaks related to S, G, H, pcoumaric acid (pCA), ferulic acid (FA), and tricin (T) are all
well defined in the aromatic region.42 Condensed S and G
signals were also found in the CELF lignin, which are
commonly observed from the lignin isolated by cosolvent
pretreatment at temperature conditions of 180 °C or higher
with acid.43−45 However, there exist dramatic differences
between the original CELF lignin and modified lignin in both
the aromatic and aliphatic regions. Specifically, it was found
that the cross peaks associated with G and H lignin units were
significantly altered during the Mannich reaction, while the
signal of the S lignin units remained relatively stable. In
addition, there are two Mannich reactive sites in tricin namely
T6 and T8, which also disappeared in the aminated lignin
because of the reaction of DETA and formaldehyde at these
activate aromatic sites. Several intense signals were observed in
75 °C, 4 h, 4 mmol DETA, 16 mmol formal aldehyde,
0.2 mL acetic acid
a
For each run, 200 mg of lignin was dissolved in 2 mL of dioxane.
Temperature A1−A4: 45, 60, 75, and 90 °C; time B1−B4: 1, 2, 3,
and 4 h; DETA content C1−C4: 4, 8, 12, and 16 mmol;
formaldehyde content D1−D4: 4, 8, 12, and 16 mmol; acetic acid
content E1−E4: 0.1, 0.2, 0.3, and 0.4 mL. cK: average value of each
factor at different levels. dR: extremum of each factor.
b
content (D) < reaction temperature (A), according to the
extremum of each factor (R value). The reaction temperature
was found to be the most important factor, and the extent of
amination appears to achieve its maximum at 75 °C. This
could be because the Mannich reaction is an endothermic
reaction thus it can be promoted by increasing the temperature. A further increase in temperature has been shown to
have a negative effect on animation, which could be because of
unnecessary formaldehyde and DETA evaporation, thus
resulting in a decrease of reaction efficiency.36 In conclusion,
the optimal combination parameters of the experiment are 75
°C, 4 h, 4 mmol DETA, 16 mmol formaldehyde, and 0.20 mL
acetic acid, according to the average values of each factor at
different levels (K value). A large scale batch reaction was then
performed at this optimal condition, and the obtained
aminated CELF lignin was subsequently characterized by
several state-of-the-art analytical techniques.
FTIR Analysis. The structural characteristics of the CELF
lignin and aminated lignin were analyzed by FTIR as shown in
Figure 2. Results showed that the aminated CELF lignin
exhibited some basic adsorption peaks of CELF lignin, which
indicates that the skeleton structure of lignin remains basically
intact during the Mannich reaction. For example, hydroxyl
group stretch (3400 cm−1), asymmetrical stretching vibrations
of −CH3 and −CH2− (2937 cm−1), and symmetrical
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Figure 3. HSQC analysis of the original CELF lignin and its aminated product. (A) CELF lignin aromatic region; (B) aminated lignin aromatic
region; (C) CELF lignin aliphatic region; (D) aminated lignin aliphatic region.
the CELF lignin at δC/δH 178/9.5−9.6 ppm and δC/δH 123/
7.5 ppm, representing the aldehyde (Cα) and furanic C−H
(C3) signals of the five-substituted furfural derivatives,
respectively.41 It has been reported that these types of furfural
derivatives such as 5-hydroxymethyfurfural or 5(methoxymethyl)furfural, which arose from sugar dehydration
reactions could be condensed with the lignin structure during
the acid catalyzed organosolv pretreatments.46 These types of
structure were absent after the Mannich reaction possibly
because of the reaction between aldehydes and primary amines
to form Schiff bases.40
In the aliphatic region, lignin interlinkages especially the β−
O−4 linkages were dramatically cleaved during the CELF
pretreatment process, and in fact, these linkages could be only
detected at a noise level (data not shown). This is consistent
with previous studies that reported CELF pretreatment
performed at high severities (180 °C) was capable of achieving
near-complete removal of its native β-aryl ether linkages
without hydrogen input or further heterogeneous catalytic
processing.31,47 This process is expected to generate a
substantial amount of phenolic hydroxyl groups that favor
the subsequent amination process. The methoxyl group
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Figure 4. Quantitative 31P NMR spectra of the (A) CELF lignin and (B) aminated lignin.
Figure 5. SEM images of lignin (top) and aminated lignin (bottom). (A,D): Mag. = 5k; (B,E): Mag. = 10k; (C,F): Mag. = 20k.
P NMR Analysis. Quantitative 31P NMR technique was
further employed to determine different types of hydroxyl
groups including aliphatic, phenolic, and carboxylic acid in the
CELF lignin and aminated lignin, and the results are shown in
Figure 4. The phosphitylation reaction of various OHs in lignin
structural units with TMDP is shown in Figure S5. According
to a recent study, the S hydroxyl group and condensed G
hydroxyl groups are not fully baseline resolved and therefore
are combined into C5 substituted hydroxyl groups in this study
to avoid any possible overestimation of S and underestimation
of the G condensed unit.48 As compared to the original CELF
lignin, a noticeable decrease in phenolic G and H hydroxyl
groups was observed in the aminated CELF lignin after
Mannich reaction. By contrast, the content of phenolic C5
substituted hydroxyl groups including the S and condensed G
and H units were considerably higher in the modified lignin
than that in the original CELF lignin. The 31P results also
indicated that the reactivity of the reaction sites in G lignin
units was higher than that in the H lignin, and as a result, the
proportion of the G unit decreased more obviously than the H
unit. The slight loss of aliphatic hydroxyl group may result
31
remains as the pronounced functional group in both lignin
spectra. Compared to the unmodified CELF lignin, a
significant amount of new signals appeared in the phenolic
lignin side chain of aminated lignin, which was mainly ascribed
to the methylene bridge of DETA introduced during the
Mannich reaction. Our HSQC analysis also reveals that both
the primary and secondary amines are capable of activating the
formaldehyde. As shown in Figure 3, there still exist plenty of
secondary amines in our proposed aminated lignin structure,
which means that as new formaldehyde is activated by these
protons, additional reactive phenolic G and H units could be
subsequently grafted onto these partially aminated lignins until
all the protons on the N atoms are replaced. A schematic
diagram of Mannich reaction and the structural transformation
of CELF lignin during the amination reaction is shown in
Figure S3. The unmodified and aminated lignin was also
subjected to a qualitatively visual ninhydrin test (Figure S4).
The original CELF lignin had a negative/orange color
indicating the absence of amines, while the change of color
in the aminated lignin proved the existence of primary and
secondary amine groups.
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from the possible loss of hydrophilic lignin fragment during the
dialysis of aminated lignin.
SEM Analysis. The modified lignin sample was obtained by
rotatory evaporation and centrifugation followed by extended
dialysis and freeze-drying. After repeated vacuum evaporation
and centrifugation, the formation of a turbid suspension
suggested the possible presence of aminated lignin nanoparticles. The morphological changes of CELF lignin during
Mannich reaction are monitored by SEM and displayed in
Figure 5. The unmodified CELF lignin in the solid state has a
much larger particle size compared to the aminated lignin and
appears granulated with irregular grains of compact structure.
The surface of aminated lignin is much smoother than that of
the original lignin. The nanospheric particles also aggregated
into a micron-sized cluster with undefined shapes in the
aminated lignin, which were possibly induced by the freezedrying process.49
Thermal Gravimetric Analysis. The first derivative of the
thermogravimetric (DTG) curves of the original CELF lignin
and its aminated products exhibited different thermal
degradation stages in N2 (Figure 6). Overall, the aminated
Article
Table 2. Surface Area and Pore Volume of the CELF Lignin
and Aminated Lignin as Determined by Physisorption
Analysis
sample
BET surface area
(m2/g)
BJH pore volume
(cm3/g)
CELF lignin
aminated CELF lignin
4.2
5.9
0.002
0.006
mesoporosity of these lignin samples is also confirmed by the
pore size distribution analysis (Figure S6). Because the
aminated lignin had a larger surface area, it provided more
adsorption sites, and hence enhanced dye removal can be
anticipated.
Removal of Dye by the Adsorbent. To demonstrate the
potential dye-adsorption property of the modified CELF lignin,
two types of dyes were tested in the lignin-dye adsorption
experiment: a cationic dye MB and an anionic azo dye DB 1.
The qualitative and quantitative effect of lignin loading on the
dye decolorization efficiency for the original and aminated
lignin is shown in Figure 7. These results indicated that the
aminated lignin showed drastically improved decolorization
efficiency for both dyes especially the anionic DB 1 dye. For
example, the amination process increased the DB 1 dyeremoval efficiency from <5 to >90% even at extremely low
lignin loadings. The decolorization efficiency for MB dye is
proportional to the dose of lignin, while no correlation could
be obtained between the efficiency of DB 1 dye removal and
the concentration of lignin. This might be attributed to the
unmodified lignin inability to adsorb DB 1 dye even at
extremely high lignin dose and the aminated lignin strong
ability to adsorb DB 1 dye even at low lignin loadings. The
effect of amino content on the adsorptivity of the aminated
CELF lignin was further investigated, and the results indicated
that the dye decolorization efficiency was positively correlated
to the amino content of the modified CELF lignin (Figure S7).
The modified lignin has much higher decolorization efficiency
toward the anionic dye (DB 1) compared to the cationic dye
(MB). This could be mainly because of the electrostatic
coupling between the cationic side chain (amine group) of the
aminated lignin and the anionic sites of the DB 1 dye.52 It is
well known that pH affects the adsorption of most organic
pollutants as well as the surface charges of adsorbents. To
further confirm that electrostatic interactions are key
mechanism of adsorptive removal of dyes in DB 1 dye in
aqueous solutions, the effect of initial pH on the dye
decolorization efficiency and zeta potential of aminated
CELF lignin was evaluated within the pH range of ∼4.0 and
10.0 (Figure 8). At extreme acidic or basic conditions (pH < 3
or > 12), lignin samples were found to be partially or
completely dissolved in aqueous solutions, thus their
adsorption and surface charge behaviors were not investigated
at these conditions. These results also indicated that high pH
values resulted in a decrease in the adsorptivity of aminated
CELF lignin, which could be because of the increase of the
magnitude of the negative zeta potential as pH increases. The
aminated CELF lignin has a point of zero charge (pHPZC)
around 4.5, and its magnitude of zeta potential at each tested
pH value (−2 to −25 mV) is significantly lower than that of
the unmodified CELF lignin ranging from −68 to −75 mV
(Figure 8). This is because of the addition of the cationic
amine group onto the side chain of aminated lignin. DB 1 dye
has four sulfonate groups, thus it remains negatively charged at
Figure 6. Derivative thermogravimetric curves of the CELF lignin and
aminated lignin.
CELF lignin degraded faster than the unmodified lignin. In the
pyrolysis range (200−600 °C), the DTG curve for the CELF
lignin exhibits a single decomposition step with a decomposition peak temperature of 395 °C, while the curve for the
aminated CELF lignin exhibits two composition steps, which
are around 243 and 317 °C. The decomposition step above
300 °C is probably because of the cleavage of the C−C
linkages and the demethoxylation of aromatic ring.37,50 The
lower degradation temperature for aminated lignin is probably
because of the lower C−N bond energy compared to the C−C
bond. The decomposition step around 243 °C could be
because of the degradation and evaporation of small molecular
weight lignin fragments such as aliphatic side chains. The final
degradation step around 600 °C is probably because of the
crack of C−C/H bond of the charcoal that was formed during
the pyrolysis.51
BET Surface Area Analysis. Table 2 summarizes the BET
specific surface areas (SBET) and Barrett−Joyner−Halenda
(BJH) pore volumes (VBJH) of the lignins before and after
amination. Results indicated that the aminated lignin exhibited
higher SBET and VBJH than those of the original CELF lignin.
For example, the SBET values were 4.2 and 5.9 m2/g for the
CELF lignin and aminated lignin, respectively. In addition, the
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Figure 7. Dye adsorption capacity of the CELF lignin and aminated lignin. (A) Effect of lignin loading on dye decolorization efficiency. (B) MB
dye before (1) and after (2, 3) 24 h lignin adsorption. (C). DB 1 dye before (1) and after (2, 3) 24 h lignin adsorption.
adsorption at lower pH. Furthermore, the decolorization
efficiency is still above 60% even at high pH, although there
exists substantial electrostatic repulsion between the modified
lignin and DB 1 dye. This indicates that mechanisms other
than electrostatic interaction such as hydrogen bonding and
π−π stacking are also operative for the DB 1 dye adsorption on
the aminated CELF lignin.54 On the other hand, hydrogen
bonding, π-interaction, and limited electrostatic interaction
between lignin dissociated carboxyl/hydroxyl groups and the
cationic sites of the dye molecule are believed to be responsible
for the MB dye adsorption.1 Figure 9 shows a proposed
scheme for the MB and DB 1 dye binding to the unmodified
CELF lignin and aminated lignin surface.
The Langmuir and Freundlich model was used to study the
adsorption isotherms of azo-dyes, and their equations (eqs 1
and 2) are shown below
Figure 8. Effect of pH on the zeta potential and adsorptivity of the
modified lignin toward DB.
basic conditions and even at highly acidic solutions as these
protonated sulfonate groups have a pKa value lower than
zero.53 Therefore, the increase of the net negative zeta
potential could further cause a decrease of the electrostatic
interactions between the cationic lignin side chain and the
negatively charged DB 1 dye in aqueous solution. This suggests
that electrostatic force is a major interaction for DB 1 dye
Ce
C
1
= e +
Qe
Qm
Q mKL
log Q e = log KF +
1
log Ce
n
(1)
(2)
Figure 9. Proposed scheme of DB and MB dye binding to the CELF lignin (A) and aminated lignin (B).
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Figure 10. Adsorption isotherms of DB 1 by aminated CELF lignin. (A) Langmuir fitted adsorption isotherm curve; (B) linear fit of the Langmuir
model (R2 = 0.99); (C) Freundlich fitted adsorption isotherm curve; (D) linear fit of the La Freundlich model (R2 = 0.81).
where ce (mg/mL) is the equilibrium concentration, Qe (mg/
g) is the equilibrium adsorption capacity, Qm (mg/g) is the
maximum adsorption capacity of the Langmuir isotherm
model, KL (mL/mg) is a Langmuir adsorption coefficient, KF
(mL/mg) is the Freundlich constant, and 1/n is an indicator
that reflects the nonlinear degree of adsorption. Figure 10
shows the Langmuir (A) and Freundlich (C) adsorption
isotherms curves, and the linear analysis (B and D) indicated
that the observed dye adsorption data for aminated CELF
lignin were better described by the Langmuir isotherm model
as confirmed by the higher coefficient R2. The Langmuir fitting
results suggested that the adsorption process between the
lignin and the azo dye could be characterized as a monolayer
type of adsorption. The Langmuir isotherm analysis indicated
that the maximum adsorption capacity of the aminated CELF
lignin is 502.7 mg/g for DB 1 dye. A direct comparison of the
maximum adsorption capacities of different anionic azo dyes
on various previously reported adsorbents is presented in
Table 3. Based on our literature survey, it was found that the
adsorption capacity of aminated CELF lignin is higher than
that of all other adsorbents except carbon nanospheres. It
should be noted that these adsorbents include commercial
activated carbon, anion exchange membrane, and multiwalled
carbon nanotubes, which are all well-known for their high
aspect ratio, natural porosity, and strong ability to adsorb
pollutants from the aqueous system. This suggests that the
aminated CELF lignin, as a low-cost renewable resource, is
highly suitable for the removal of azo-dyes from the aqueous
solutions.
The rate of the adsorption process is an important factor
that determines if the sorbent could be used on large scales in
industrial applications and it can be determined by kinetic
studies. In the study of solid−liquid static adsorption kinetics,
the relationship between the time and the amount of
adsorption is typically fitted through dynamic models.64 Two
Table 3. Adsorption Performance of Different Adsorbents
toward Azo-Dyes as Characterized by the Maximum
Adsorption Capacity (mg dye/g Substrate)
adsorbent
dye adsorbate
maximum capacity
(mg/g)
granular activated carbon
zeolite
graphene oxide
chitosan halloysite
nanotubes
multiwalled carbon
nanotube
Mn0.4Zn0.6Fe2O4
nanoparticles
Mn0.4Zn0.6Fe2O4
nanoparticles
SEG-modified starch
SEG-modified starch
lignin amine-coated
Fe3O4
chitosan
multiwalled carbon
nanotube
graphene oxide sponge
aminated CELF lignin
chitosan-based hydrogel
Congo red
DB 71
acid orange 8
Congo red
9.1
13.7
29.0
41.5
55
6
11
56
tartrazine
84.0
57
tartrazine
90.8
58
Ponceau 4R
101.4
58
direct red 23
acid blue 92
acid scarlet
GR
tartrazine
DB 53
129.9
147.1
176.5
59
59
54
350
409.4
60
53
501.3
502.7
520
5
present
61
555.6
689.7
62
63
carbon nanospheres
Fe(OH)3@cellulose
hybrid fibers
direct red 80
DB 1
erichrome
black T
acid red 88
Congo red
references
mathematical models are usually adapted to analyze the
dynamic models of adsorption process, namely pseudo-firstorder (eq 3) and pseudo-second-order (eq 4) equation
K1t
log(Q e − Q t ) = log Q e −
(3)
2.303
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Figure 11. Pseudo-second-order plot (A) and pseudo-first order plot (B) for the DB dye adsorption kinetics by the aminated CELF lignin.
t
t
1
=
+
Qt
Qe
Q e 2K 2
recycle performances of aminated CELF lignin for DB 1 dye
removal were also investigated in this study. The results, as
shown in Figure 12, show that no significant reduction in the
(4)
where Qe and Qt are adsorption capacity (mg/g) at equilibrium
time and any instant of adsorption time t (min) and K1 and K2
are the rate constant of the pseudo-first-order and secondorder adsorption, respectively. The study about the effect of
time on the adsorption process, as shown in Figure S8,
suggests that the amount of dye adsorbed by the adsorbent
increases rapidly at the beginning for 30 min, and then the
adsorption efficiency becomes slow for 240 min until the
adsorption equilibrium is reached. The pseudo first and second
kinetic models assume that the adsorption process is governed
by diffusion and chemical adsorption mechanism, respectively.
The observed experimental data were fitted to the pseudo-firstorder and second-order equations, and their linear fitted plots
and the parameters of the kinetic model are shown in Figure
11 and Table 4, respectively. It was found that the correlation
Figure 12. Removal efficiency of the aminated CELF lignin for DB 1
dye after four adsorption−desorption cycles.
Table 4. Parameters of the Adsorption Kinetic Model
kinect model
pseudo-first order
pseudo-second
order
qe
(mg g−1)
K1 (min−1) or K2
(g mg−1 min−1)
R2
285.8
511.7
0.01152
0.00013
0.984
0.996
adsorption efficiency is found for three cycles compared with
that of the fresh adsorbent, although there is a gradual decrease
in the dye-removal efficiencies possibly because of the
incomplete of dye desorption. The dye-removal efficiency
decreased to 65% for the fourth use, probably because of the
saturation of the adsorbent surface. Thus, the recycle study
demonstrated that the aminated CLEF lignin remained as an
efficient adsorbent even after multiple reuses.
Aminated CELF Lignin Characterization after Dye
Adsorption. The FTIR spectra of aminated CELF lignin
before and after dye adsorption are shown in Figure S10. After
adsorption, two additional peaks at around 1200 and 1035
cm−1 representing the stretching vibration bands of the
sulfonate group appear in the aminated lignin because of the
attachment of the DB 1 dye. The NN stretching vibration
from DB 1 dye is unclear in the IR spectra because the direct
dye is a symmetrical trans azo compound. SEM was also used
to analyze the morphology of the aminated CELF lignin
surface after the adsorption of DB 1 dye (Figure 13). Results
showed that the shape of lignin particles did not change
dramatically and remained as an aggregated cluster with
irregular shapes and heterogeneous surface, but their size
appeared to increase significantly after dye adsorption. These
observations clearly revealed that the DB 1 dye is adsorbed on
the surface of the modified CELF lignin.
coefficient (R2) of pseudo-second-order equation kinetic
model was higher than that of the pseudo-first-order model,
suggesting that the pseudo-second-order kinetic model may be
more suitable for describing the kinetics of the adsorption
process of the DB 1 dye on the aminated CELF lignin. This
suggested that the adsorption behaviors of azo-dye onto the
modified lignin are dominated by chemical adsorption instead
of diffusion process. It has been reported that the adsorption
process is generally divided into three main stages, including
the film diffusion stage, intraparticle diffusion stage, and the
final actual adsorption stage.65 To further test if the
intraparticle diffusion is the only rate determining step of the
adsorption process, the intraparticle diffusion kinetic model is
also fitted with the experimental data (Figure S9), and results
indicated that the adsorption process has more than one speedcontrolling step. In addition, the relatively large boundary layer
thickness as reflected by the large intercept of the linear plot
(t1/2 vs qt) suggests that membrane diffusion might also have a
great effect on the adsorption process.65
Reusability of the adsorbent represents an important aspect
to minimize the cost of the overall adsorption process. The
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phase by first neutralization with ammonium hydroxide
followed by THF evaporation and subsequent vacuum
filtration of the precipitated lignin from the neutralized liquor.
The obtained CELF lignin was washed with water and diethyl
ether and dried at 45 °C in an incubator. Once dried, the lignin
was finally crushed to a fine powder in a mortar and pestle and
stored in a container prior to further test and modification.
Amination of the CELF Lignin. The corn stover CELF
lignin (∼200 mg) was mixed with 2 mL of dioxane in a round
flask under constant stirring for 20 min at room temperature
until the lignin was fully dissolved. Specified amounts of
DETA, acetic acid, and formaldehyde were then added into the
solution with continuous stirring. Formaldehyde was added
stepwise to avoid unnecessary crosslinking reactions. Subsequently, the flask was heated in a sand bath, kept at a
specified temperature (45, 60, 75, and 90 °C), and stirred for a
specified time (1, 2, 3, and 4 h). Afterward, the reaction
mixture was evaporated under reduced pressure to remove the
majority of the organic solvent followed by dialysis with a
molecular weight cut off of 1000 Da. The obtained aminated
CELF lignin was finally freeze-dried and stored at room
temperature before further characterization.
Lignin Characterization before and after Amination.
FTIR Analysis. The IR spectra were collected using a Spectrum
One FTIR spectrophotometer (PerkinElmer, Wellesley, MA)
equipped with a diamond-composite attenuated total reflectance cell from 1000 to 4000 cm−1 with 128 scans at 4 cm−1
resolutions.
NMR Analysis. NMR experiments were acquired with a
Bruker Avance III HD 500 MHz spectrometer equipped with a
5 mm N2 cryogenically cooled BBO H&F probe, according to
previously published literatures.32,66 A standard Bruker pulse
sequence (hsqcetgpspsi2.2) and an inverse-gated decoupling
pulse sequence (Waltz-16) were applied for HSQC and 31P
NMR experiment, respectively.
SEM Analysis. The morphology of lignin samples was
observed with a scanning electron microscope (Zeiss Auriga,
Germany) at an accelerating voltage of 5 kV. The samples were
sputter-coated with Au using an SPI-Module sputter coater for
50 s. Imaging was subsequently captured at various
magnifications from 2k to 20k.
Thermal Gravimetric Analysis. The TGA was performed by
a TGA Q50 thermo-gravimetric analyzer (TA instruments,
UDA). Lignin samples (∼5 mg) were loaded to a platinum
sample pan (TA instruments) and heated in nitrogen from 25
to 105 °C at 20 °C/min. After incubating at 105 °C for 10
min, it was heated further from 105 to 800 °C at a heating rate
of 20 °C/min.
Zeta Potential Analysis. The zeta potential of lignin
suspensions was measured at different hydrogen ion
concentrations while keeping a concentration of 1 mg/mL,
using ZetaPALS (Brookhaven Instruments Corporation, NY).
The mean zeta potential of each suspension was calculated
from 10 measurements.
Surface Area and Pore Size Analysis. The N2 adsorption−
desorption measurement of samples was carried out at 77 K on
a Quantachrome Autosorb iQ. The samples were first degassed
at 353 K for ∼17 h before being loaded into the analysis
station. The pore volume and pore size distribution were
determined using a BJH method. The specific surface area was
calculated using BET in the P/P0 range of 0.05−0.30.
Dye Decolorization Study. Various amounts of CELF
lignin and aminated lignin were mixed with 25 mL of MB or
Figure 13. SEM images of the aminated lignin after adsorption of DB
1 dye. [(A) Mag. = 2k. (B) Mag. = 5k. (C) Mag. = 10k. (D) Mag. =
20k].
CONCLUSIONS
Development of low-cost renewable bioadsorbents for removal
of toxic dyes from contaminated water has been a topic of great
interest but remains challenging. In this study, the aminated
corn stover lignin was synthesized via combinatorial CELF
pretreatment and Mannich reaction to remove azo dyes from
aqueous solutions. Under acid conditions, both the primary
and secondary amines have high reactivity toward the H3/5 and
G5 position of CELF lignin. SEM revealed that the original
lignin particles are distributed in a large conglomerate, while
the surface of aminated lignin becomes smoother, and the
nanospherical particles of the modified lignin aggregate into
nano- and micron-sized cluster with undefined shapes. The
combination of CELF pretreatment and Mannich reaction
significantly increased the adsorption behavior of aminated
lignin toward azo DB 1 dye with a maximum capacity of 502.7
mg/g, which is significantly higher than that of many adsorbent
materials reported in the literature. Recycle studied suggested
that once recovered, the bioadsorbent was capable of
maintaining a relatively high dye removal efficiency (>85%)
even after three recycles. In conclusion, the proposed aminated
CELF lignin could be considered as a cost-effective
bioadsorptive platform for the efficient removal of azo dyes
from aqueous solutions.
■
EXPERIMENTAL SECTION
Feedstocks and Chemicals. The Kramer corn stover was
provided by the National Renewable Energy Laboratory
(NREL, Golden, CO). The corn stover was knife-milled to
pass through a 1 mm particle size interior sieve using a
laboratory mill (model 4, Arthur H. Thomas Company,
Philadelphia, PA). All the chemicals used in this study were
used as received from Sigma-Aldrich without any further
purification.
Production of the CELF Lignin. CELF pretreatment of
corn stover was performed in a custom built 1 L Hastelloy Parr
reactor (Parr instruments Company, Moline, IL) at 7.5 wt %
solids loading and 0.5 wt % H2SO4 acid loading. The CELF
reaction was sustained at 180 °C for 25 min in an equivolume
mixture of THF and water. After pretreatment, the reactor was
quenched in a 25 °C water bath, and the liquid phase was
separated from the pretreated solids through vacuum paper
filtration. The CELF lignin was then isolated from the liquid
■
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Knoxville, Tennessee 37996, United States; Biosciences Division,
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831,
United States; Department of Forestry, Wildlife, and Fisheries;
Center for Renewable Carbon, The University of Tennessee
Knoxville, Institute of Agriculture, Knoxville, Tennessee 37996,
United States; orcid.org/0000-0002-3536-554X;
Email: argauskas@utk.edu
DB 1 dye solution with a concentration of 50 mg/L. The
mixture was left in an incubator at 25 °C and 150 rpm for 24 h.
The concentration of dye in the supernatant of the solution at
the equilibrium was determined by a UV spectrophotometer.
The amount of dye adsorbed (qe) by lignin substrates was
calculated based on the following eq 5
qe =
v(Co − Ce)
m
(5)
Authors
where co and ce represent the initial and equilibrium
concentrations of dye solution (mg/L), respectively, v is the
volume of the total solution (mL), and m is the dry weight of
the lignin sample (g). The maximum wavelength for MB and
DB 1 dye was set at 663 and 624 nm, respectively. The
extinction coefficient of MB and DB 1 dye was determined to
be 149.3 and 12.3 L mol−1 cm−1 based on the Beer’s law
calibration (Figures S1 and S2). The decolorization efficiency
(η) is defined by
η=
Co − C
× 100%
Co
Brent Scheidemantle − Center of Environmental and Research
Technology (CE-CERT) and Department of Chemical and
Environmental Engineering, Bourns College of Engineering,
University of California, Riverside, California 92507, United
States
Mi Li − Department of Chemical & Biomolecular Engineering,
University of Tennessee Knoxville, Knoxville, Tennessee 37996,
United States; orcid.org/0000-0001-7523-1266
Yun-yan Wang − Department of Forestry, Wildlife, and
Fisheries; Center for Renewable Carbon, The University of
Tennessee Knoxville, Institute of Agriculture, Knoxville,
Tennessee 37996, United States
Xianhui Zhao − Chemical Sciences Division, Oak Ridge
National Laboratory, Oak Ridge, Tennessee 37831, United
States
Miguel Toro-González − Isotope and Fuel Cycle Technology
Division, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831, United States
Priyanka Singh − Center of Environmental and Research
Technology (CE-CERT) and Department of Chemical and
Environmental Engineering, Bourns College of Engineering,
University of California, Riverside, California 92507, United
States
Yunqiao Pu − Biosciences Division, Oak Ridge National
Laboratory, Oak Ridge, Tennessee 37831, United States
Charles E. Wyman − Center of Environmental and Research
Technology (CE-CERT) and Department of Chemical and
Environmental Engineering, Bourns College of Engineering,
University of California, Riverside, California 92507, United
States
Soydan Ozcan − Department of Mechanical, Aerospace,
Biomedical Engineering, University of Tennessee, Knoxville,
Tennessee 37996, United States; Manufacturing Demonstration
Facility, Energy and Transportation Science Division, Oak Ridge
National Laboratory, Knoxville, Tennessee 37932, United
States
Charles M. Cai − Center of Environmental and Research
Technology (CE-CERT) and Department of Chemical and
Environmental Engineering, Bourns College of Engineering,
University of California, Riverside, California 92507, United
States; orcid.org/0000-0002-5047-0815
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.9b03717
(6)
where co represents the initial concentration, and c is the
concentration of dye in the supernatant after decolorization.
The adsorption isotherms of DB 1 dye onto aminated lignin
were measured with varying concentrations of dye ranging
from 0.05 to 1 mg/mL at 25 °C. To further investigate the
kinetics of dye adsorption, the equilibrium concentrations of
dye solution were measured from 5 min to 8 h after mixing 20
mg of aminated lignin with 40 mL of DB 1 with an initial
concentration of 1 mg/mL. The effect of initial pH on dye
adsorption was studied by mixing ∼10 mg of lignin with 25 mL
of initial DB 1 dye with a concentration of 50 mg/mL at 25 °C.
∼0.1 M HNO3 and 0.01 M of NaOH were used to adjust the
pH between 4 and 10. For the recycling experiment, dilute
NaOH (pH = 10) was used to release the adsorbed dye from
the adsorbent. The adsorption of the dye by the regenerated
lignin was repeated four times by mixing ∼20 mg of solid with
25 mL of MB or DB 1 dye solution with a concentration of 50
mg/L.
■
Article
ASSOCIATED CONTENT
* Supporting Information
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsomega.9b03717.
Calibration curve of dyes; schematic diagram of
Mannich reaction; ninhydrin test of lignin amination;
phosphitylation reactions between lignin OHs and
TMDP; pore size distribution of lignin samples; effect
of amino content on the adsorptivity of aminated CELF
lignin; effect of contact time on adsorption; intraparticle
diffusion kinetic of DB 1 dye; and FTIR spectra of the
aminated CELF lignin before and after dye adsorption
(PDF)
Author Contributions
■
AUTHOR INFORMATION
The experiments were performed through contributions of all
Corresponding Authors
Xianzhi Meng − Department of Chemical & Biomolecular
Engineering, University of Tennessee Knoxville, Knoxville,
Tennessee 37996, United States; orcid.org/0000-00034303-3403; Email: xmeng5@utk.edu
Arthur J. Ragauskas − Department of Chemical &
Biomolecular Engineering, University of Tennessee Knoxville,
the authors. All the authors have approved the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work is supported by “Agriculture and Food Research
InitiativeSustainable Bioenergy and Bioproducts Challenge
Area” [grant no. USDA-NIFA-AFRI-006352/project accession
no. 1015189] from the U.S. Department of Agriculture
(USDA) National Institute of Food and Agriculture (NIFA).
Facilities at UC Riverside were provided by the Bourns’
College of Engineering Center for Environmental Research &
Technology (CE-CERT). We also want to thank Sarah
Humphries from the University of TennesseeKnoxville for
her administrative support including help us purchasing all the
suppliers for the study.
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Article
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