Competitive adsorption of three phenolic compounds to hydrophilic urea-formaldehyde macroporous foams derived from lignin-based Pickering HIPEs template

Article abstract of DOI:10.1039/c6ra20919j

Hydrophilic urea-formaldehyde macroporous foams (UFMF) were simply synthesized by templating oil-in-water Pickering high internal phase emulsions (HIPEs) solely stabilized by lignin particles. Morphology and structure of the UFMF can be easily tailored by varying the experimental compositions of Pickering HIPEs and polymerization time. Due to their porous matrix, hydrophilic surface, and high content of binding groups (such as hydroxyl groups and carbonyl groups), as-prepared UFMF was adopted for the removal of three phenolic compounds (PCs) from aqueous solution. The influences of pH, solution temperature, contact time, and initial concentration of 2,4,5-trichlorophenol (2,4,5-TCP), 2,4,6-trichlorophenol (2,4,6-TCP) and 4-nitrophenol (4-NP) on adsorption capacity and competitive adsorption of multi PCs were investigated via the batch mode experiments. The adsorption kinetics and adsorption isotherms fit well by a pseudo-second-order model and Langmuir model, respectively. The max adsorption capacity of 2,4,5-TCP, 2,4,6-TCP and 4-NP at 25 °C are 116.3 mg g^-1, 78.74 mg g^-1, 29.41 mg g^-1, respectively, and their adsorption rate follows the order 2,4,5-TCP > 2,4,6-TCP > 4-NP. Moreover, the group type and position effects both influence the adsorption, and follow the order: 2,4,5-position substitution of chloride > 2,4,6-position substitution of chloride > 4-position substitution of nitro-group, showing that the substitution with the more groups has the higher adsorption affinity.

Full text of DOI:10.1039/c6ra20919j

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PAPER
Competitive adsorption of three phenolic
compounds to hydrophilic urea-formaldehyde
macroporous foams derived from lignin-based
Pickering HIPEs template†
Cite this: RSC Adv., 2016, 6, 93894
a
a
b
a
a
a
Jianming Pan,* Jialu Luo, Jun Cao, Jinxing Liu, Wei Huang, Wenli Zhang*
and Lili Yang
c
Hydrophilic urea-formaldehyde macroporous foams (UFMF) were simply synthesized by templating oil-in-
water Pickering high internal phase emulsions (HIPEs) solely stabilized by lignin particles. Morphology and
structure of the UFMF can be easily tailored by varying the experimental compositions of Pickering HIPEs
and polymerization time. Due to their porous matrix, hydrophilic surface, and high content of binding
groups (such as hydroxyl groups and carbonyl groups), as-prepared UFMF was adopted for the removal
of three phenolic compounds (PCs) from aqueous solution. The influences of pH, solution temperature,
contact time, and initial concentration of 2,4,5-trichlorophenol (2,4,5-TCP), 2,4,6-trichlorophenol (2,4,6-
TCP) and 4-nitrophenol (4-NP) on adsorption capacity and competitive adsorption of multi PCs were
investigated via the batch mode experiments. The adsorption kinetics and adsorption isotherms fit well
by a pseudo-second-order model and Langmuir model, respectively. The max adsorption capacity of
ꢀ
ꢁ1
ꢁ1
ꢁ1
2
,4,5-TCP, 2,4,6-TCP and 4-NP at 25 C are 116.3 mg g , 78.74 mg g , 29.41 mg g , respectively, and
Received 19th August 2016
Accepted 27th September 2016
their adsorption rate follows the order 2,4,5-TCP > 2,4,6-TCP > 4-NP. Moreover, the group type and
position effects both influence the adsorption, and follow the order: 2,4,5-position substitution of
chloride > 2,4,6-position substitution of chloride > 4-position substitution of nitro-group, showing that
the substitution with the more groups has the higher adsorption affinity.
DOI: 10.1039/c6ra20919j
www.rsc.org/advances
In recent years, porous and micrometer-sized materials have
gained considerable attention in the past decade as good
Introduction
Phenolic compounds (PCs), such as nitrophenols and chlor- candidates for adsorption, owing to their outstanding proper-
ophenols, are considered to be the most common organic, toxic ties such as high porosity, low density, high permeability,
5,6
and persistent pollutants in industrial and agricultural waste- simple separation and plenty active sites for stay. In the
water, they are also recognized to be the hazardous wastes and literature, various porous materials are described that show
7
the priority toxic pollutants by the US Environmental Protection excellent adsorption ability, such as porous carbon, porous
1
,2
8
9
Agency (EPA). Though a number of processes are used for PC hierarchical MgO, metal–organic framework, poly(glycidyl
10
removal from aqueous systems, adsorption is considered to be methacrylate) porous monoliths, and highly porous chitosan
one of the preferred methods due to its simplicity, high effi- monoliths. However, technologies for preparing porous
ciency, and low energy requirements. Nevertheless, adsorbents materials with high stability, controllable pore structure and
11
3
which show high capacity as well as high selectivity in combi- operational convenience are oen difficult. High internal phase
nation with a fast adsorption process are also practically emulsions (HIPEs) are known as emulsions containing more
4
unavailable.
than 74 vol% of internal phase and provide a very convenient
route to prepare macroporous polymer materials by polymer-
12,13
izing the monomers in the continuous phase.
Moreover,
macroporous polymer materials based on a polyHIPEs also
contain smaller, interconnecting pores, which are referred to as
windows connecting larger cavities. Thus, polyHIPEs are
potential adsorbents, for which the high porosity and tailored
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
2
12013, China. E-mail: zhenjiangpjm@126.com; jsdxzhwl@126.com; Fax: +86
8791800; Tel: +86 88791708
14
8
b
Yancheng Entry-Exit Inspection and Quarantine Bureau, Yancheng 210001, China
c
Key Laboratory of Functional Materials Physics and Chemistry (Ministry of interconnected porous structure are greatly desired.
Education), Jilin Normal University, Changchun 130103, China
However, a large amount of surfactants are usually needed to
form HIPEs which are undesirable especially in environmental
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra20919j
1
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protection, as common surfactants were shown to be toxic to toluene, triethanolamine, 2,4,5-TCP, 2,4,6-TCP and 4-NP were
15
living organisms. And the stabilization of conventional supplied by Aladdin reagent CO., Ltd (Shanghai, China). Water
surfactant-stabilized HIPEs presents a challenge in the forma- was freshly deionized.
16
tion process of porous polymer materials. Moreover, poly-
HIPEs stabilized by surfactants are oen a lack of sufficient
Instrumentation
17
robust properties. To debate these bottlenecks, an alternative
technology is the development of surfactant-free particle-
ꢁ
1
Infrared spectra (4000–400 cm ) was recorded on a Nicolet
NEXUS-470 FTIR apparatus (USA). The morphology of UFMF
samples were observed by scanning electron microscopy (SEM, S-
18
stabilized HIPEs, namely Pickering HIPEs. Most of the Pick-
ering HIPEs reported so far are based on hydrophobic particles
and the relevant polyHIPEs are synthesized by polymerizing the
hydrophobic monomers in the continuous oil phase of HIPEs
4800). TGA of samples were performed for powder samples
(about 10 mg) using a Diamond TG/DTA instruments (Perkin-
ꢀ
Elmer, USA) under a nitrogen atmosphere up to 800 C with
19,20
and removing the dispersed water phase.
The facts can
ꢀ
ꢁ1
a heating rate of 5.0 C min . Contact angle of UFMF11
measurement was performed on Optical Contact Angle
Measuring Device (KSV CM200). Detailed test condition was lis-
ted as follow. 2.0 uL of deionized water was injected on the
sample surface through the syringe pump. Aer the water droplet
was formed on the sample surface about 30 s, the images of the
water droplet were then obtained using the camera in measuring
device. Finally, these images were analyzed using the supplied
soware to determine the contact angle of sample.
mainly be attributed to the difficulties to obtain the particles
just with the right hydrophilicity that can be used as stabilizers
of oil-in-water (O/W) Pickering HIPEs, and search proper
hydrophilic monomers for potential application such as
21
adsorbents to bind target pollutant in aqueous solutions.
Lignin is the second most abundant bioresources on earth, it
has also been the main byproduct of the pulp and paper
22
manufacturing industry. Due to its merits of being a naturally
abundant, low-cost, and most importantly containing appro-
priate functional groups for the further modication, lignin is
nowadays emerging as a renewable starting material for the
Preparation of lignin dispersion solution
23
development of new products and processes. Very recently, Briey, the aqueous dispersions of lignin (3.0–7.5 wt%) were
our group adopted lignin particles as stabilizer to establish prepared by adding different amounts of lignin powder into
26
a
stable Pickering oil-in-water emulsion, and then the a xed amount of deionized water. Be sure to completely
composite adsorbents were successfully fabricated by radical dissolve lignin particles, the pH value of the mixture was
polymerization in the presence of functional and cross-linked adjusted by adding concentrated ammonia solution until the
24
monomers in the oil phase. Thus, lignin particles are ideal pH value was around 11. Subsequently, the resulting solution
candidate for O/W Pickering emulsiers. Urea and formalde- was ltered to remove any impurities. And the pH value of
hyde are precursors of urea-formaldehyde (UF) monomer for a stable lignin solution was reduced to 3.0 by adding HCl (1.0
the purpose of practical application due to their low cost and M). Then the lignin dispersion solution formed, and it was used
25
easy availability. Meanwhile, UF polymers based on urea- for the preparation of Pickering HIPEs templates.
formaldehyde monomer has a hydrophilic property and
a highly cross-linked calculated structure that provide a good
mechanical property. More importantly, UF polymers consist of
carbonyl and hydroxyl groups, which exhibit as an ideal
candidate for highly efficient adsorption of PCs pollutants from
aqueous solutions.
Inspired by these works, we herein report a kind of hydro-
philic urea-formaldehyde macroporous foam (UFMF) derived
from lignin-based O/W Pickering HIPEs. 2,4,5-Trichlorophenol
Synthesis of UF monomer
UF monomer was synthesized according to the work of Liu
27
et al. Reaction between melamine and formaldehyde to form
UF monomer was displayed in Scheme 1. Briey, 12 g of urea
was added into 24.5 g of 37 wt% formaldehyde in aqueous
solution under stirring with a magnetic bar. The pH of the
mixture was adjusted to 8.5 with triethanolamine, and the
ꢀ
mixture was stirred at 70 C for 1.0 h. Aer that, the reaction
mixture was cooled down to room temperature and diluted with
(
(
2,4,5-TCP), 2,4,6-trichlorophenol (2,4,6-TCP) and 4-nitrophenol
4-NP) were adopted as typical PCs to be removed by as-prepared
50 mL of water. Finally, sodium chloride (10 wt%) in respect to
UFMF adsorbents. The superb adsorption properties, such as
equilibrium, kinetics and competitive binding, were studied,
and the relationship between tuned porous structure and
excellent adsorption nature were discussed in detail.
the UF monomer solution was added prior to use.
Synthesis of hydrophilic urea-formaldehyde macroporous
foam (UFMF)
Illustration for the preparation of UFMF was shown in
Scheme 2. Pickering HIPEs were normally prepared by stirring
the continuous phase (lignin dispersion solution and UF
Experimental section
Chemicals
18
Enzymatic hydrolysis lignin was obtained from Shandong monomer) vigorously under continued stirring at 600 rpm while
Longlive Bio-technology Co., Ltd. (Shandong, China). Formal- slowly adding the toluene droplet phase. Once all of the toluene
dehyde (37 wt% aqueous solution) and urea were purchased phase had been added, the mixture was stirred at 1000 rpm
from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) viscous for a hour and homogeneous O/W Pickering HIPEs were
to be used as precursors of UF monomer. Hydrochloric acid, obtained. Subsequently, the lignin particle-stabilized HIPEs
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Scheme 1 The reaction between urea and formaldehyde to form UF monomer and UFMF.
The kinetics adsorption experiments were also measured at
ꢀ
2
5 C, and the pH of testing solution was at 6.0, respectively. At
the desired time intervals (5.0 min to 720 min), the testing
solutions were ltered to analyze the residual concentration
t
(C ). The adsorption amount of 2,4,5-TCP, 2,4,6-TCP, 4-NP at
ꢁ
1
t
time t, Q (mg g ), was calculated from the eqn (1).
ðC
0
ꢁ C ÞV
t
Q
t
¼
(1)
W
where V is the volume of testing solution (V is equal to 10 mL),
and W (g) is the weight of dried UFMF11.
To gain insight into the adsorption equilibrium, the initial
concentration of 2,4,5-TCP, 2,4,6-TCP was ranged from 20 mg
Scheme 2 Schematic of the formation of UFMF with open-cell
structure.
18
ꢁ
1
ꢁ1
L
to 100 mg L , while the initial concentration of 4-NP was
ꢀ
were transferred into an oven and polymerized at 70 C for 3.0 h.
The products were puried with ethanol to remove the internal
toluene, and then UFMF were obtained when they were air-
dried at room temperature for 24 h. The detailed experi-
mental compositions are shown in Table 1.
ꢁ1
ꢁ1
ranged from 10 mg L to 50 mg L . UFMF11 were weighed
and immerged in testing solutions with various initial concen-
trations under continuous stirring for 24 h, respectively. The
same analysis method as mentioned above was employed to
detect the initial and nal concentrations by Vis spectropho-
ꢁ
1
tometer. The equilibrium adsorption amount Q
e
(mg g ) was
calculated based on eqn (2).
Batch mode adsorption experiments
ðC
0
ꢁ C ÞV
e
The effect of different initial solution pH and temperature to the
adsorption of 2,4,5-TCP, 2,4,6-TCP and 4-NP by UFMF11 was
Q
e
¼
(2)
W
ꢁ
1

rstly investigated, and the range of pH was 3.0 to 8.0. The
where C
or 2,4,6-TCP and 4-NP).
Another set of binary binding experiments were conducted
e
(mg L ) is the equilibrium concentration of 2,4,5-TCP
initial concentration of 2,4,5-TCP and 2,4,6-TCP in each
(
ꢁ
1
aqueous solution was 100 mg L , while the initial concentra-
ꢁ1
tion of 4-NP was 50 mg L . 10 mg of UFMF11 was immerged in
with sequential addition of 2,4,6-TCP, 4-NP, and 2,4,6-TCP + 4-
NP into each testing solutions of 2,4,5-TCP. The pH of testing
solution was xed at 6.0, and the weight of UFMF11 was 10 mg.
Aer the batch mode adsorption for 12 h, the residual
concentrations of 2,4,5-TCP was determined by high perfor-
mance liquid chromatography (HPLC). The injection loop
ꢀ
ꢀ
testing solutions under continuous stirring at 25 C (or 15 C
ꢀ
and 35 C) for 12 h. Aer the desired time, the supernatant was
obtained by centrifugal separation, and the concentrations of
2,4,5-TCP, 2,4,6-TCP, 4-NP were measured by UV-vis spectro-
photometry at wavelength of 292 nm, 288 nm, 252 nm,
respectively. Finally, the removal percentage (%) was calculated.
Table 1 The detailed experimental compositions of Pickering HIPEs for synthesizing of UFMF
Aqueous phase
Lignin
dispersion (mL)
UF monomer
(mL)
Monomer
fraction (%)
Oil phase
(toluene) (mL)
Inter phase
fraction (%)
Polymerization
time (h)
Sample
Lignin (wt%)
HIPEs-1
HIPEs-2
HIPEs-3
HIPEs-4
HIPEs-5
HIPEs-6
HIPEs-7
HIPEs-8
HIPEs-9
HIPEs-10
HIPEs-11
5.0
5.0
5.0
5.0
7.5
7.5
7.5
3.0
7.5
7.5
7.5
3.0
1.8
1.2
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
3.0
4.2
4.8
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
50
70
80
90
90
90
90
90
90
90
90
24
24
24
24
18
24
34
24
24
24
24
80
80
80
80
75
80
85
80
80
80
80
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
6.0
4.0
3.0
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30
volume was 20 mL, and the mobile phase was consisted of structure, which resulted in excellent mechanical performance.
methanol and deionized water with a volume ratio of 70 : 30. Further, from the Fig. 2e and f, it can be found that enhanced
ꢁ
1
The ow rate was 1.0 mL min and the column temperature deformation of the HIPEs structure and closed-cells are formed,
ꢀ
was 25 C.
due to the high viscosity of the HIPEs with increasing internal
phase fractions. But the cell size of UFMF8 (Fig. 2h) is much larger
than that of the UFMF4 (Fig. 2d) with UFMF6 (Fig. 2f), in accor-
dance with the decreased amount of lignin particles in the
external aqueous phase, owing to decreased emulsion stability in
the Pickering HIPEs template. It is possibly because that more
lignin particles at oil–water interface can cross link into network
Regeneration of UFMF
To investigate the reusability of UFMF, regeneration procedure
was conducted. The PCs-loaded UFMF11 was transferred into
the mixed solution of methanol and 0.1 M NaOH (20 mL ꢂ 3)
while stirring at 300 rpm for 2.0 h at room temperature. Then it
was washed with distilled water several times until the ltrate
was neutral. The solid was dried under vacuum at 50 C for 6.0
h. Finally, four adsorption–desorption cycles for PCs removal
were conducted as the same conditions mentioned above.
31
or aggregate closely to hinder droplets coalescence. Also, the
effect of polymerization time to the morphology was considered
in Fig. 2f and i–k, and it presents open-cells on the surface of
UFMF11, it can be partly due to the least polymerization time is
benet for forming a steric barrier for the porous structure into
ꢀ
32
a network. Void and interconnecting pore size distribution of
UFMF11 were also determined by SEM image analysis. As shown
in Fig. S2,† UFMF11 possess approximately 60% of all void pores'
diameters between 1.0 mm to 4.0 mm, and the distribution of
interconnecting pores is narrow and around 52% of pores are
between 100 mm to 150 mm. The highly porous structure of
UFMF11 can efficiently reduce the transport limitation, which is
favorable for the accessibility to the active site. Thus, UFMF11 can
be used as a promising candidate for the adsorption of phenolic
compounds in this work. The highly porous structure of UFMF11
can efficiently reduce the transport limitation, which is favorable
for the accessibility to the active site. Thus, UFMF11 can be used
as a promising candidate for the adsorption of phenolic
compounds in this work.
Results and discussion
Characterization of Pickering HIPEs and UFMF
Lignin particle-stabilized Pickering HIPEs 5–7 as shown in
Table 1 with internal phase fractions of 75–85%, respectively,
were formed. The continuous water phases consisted of 7.5 wt%
lignin particles and 5.4 mL UF monomers. The HIPEs templates
were obtained by gradual addition of different volume of
toluene to the water phases by stirring at 1000 rpm. Optical
microscopic images of Pickering HIPEs 5–7 are displayed in
Fig. 1. It can be seen that the spherical emulsion droplets are
stable, and their size distributions are polydisperse, indicating
the proper amphipathy of lignin particles to locate at the water
and oil interface. With the increase of the internal phase frac-
tion from 75% to 85%, the emulsion droplets are more crowded,
FT-IR spectra of lignin and UFMF11 are shown in Fig. 3. It is
observed that the overlapped peak of –OH and –NH
2
shis from
28
and the average sizes of the O/W droplets decrease. To identify
emulsion type, the water phase was marked by rhodamine B.
Fig. S1 (ESI†) shows the digital photographs of a drop of Pick-
ering HIPEs dispersing in water (a) and toluene (b). The drops of
Pickering HIPEs collapse immediately in water, whereas the
drops maintain the original state and oated on the surface of
toluene. These facts conrm that the formed Pickering HIPEs
ꢁ
1
ꢁ1
3390 cm to 3380 cm of UFMF11, and also the band intensity
signicantly increases with respect to unmodied lignin,
because incorporation of urea-formaldehyde (UF) polymers in
3
3
ꢁ1
the UFMF11 occurs. The band at 1705 cm is assigned to the
carbonyl group conjugated to the aromatic ring. Moreover,
unmodied lignin shows the typical and intense signals that are
ꢁ1
related to aromatic ring skeleton vibrations (1617 cm , 1515
cm , and 1428 cm ), and C–H in-plane bending vibration of
methyl and methylene. One of the signicant changes in the
FT-IR spectra is the appearance of the band at 1645 cm and
of UFMF11, which could be attributed to the
stretching vibration of –C]O and –C–N– groups, indicating
29
are the O/W emulsions.
ꢁ
1
ꢁ1
SEM images of all the UFMF samples derived from HIPEs-1 to
HIPEs-11 are listed in Fig. 2. As shown in Fig. 2a–d, HIPEs-1 and
HIPEs-2 favor the formation of UFMF with spherical capsule, and
the HIPEs-3 and HIPEs-4 create three-dimensional interconnected
UFMF. The results prove that more UF monomer may be favorable
for the efficient linkage of adjacent drops form a compact HIPEs
34
ꢁ1
ꢁ
1
1550 cm
27
the successful preparation of the shell material UF polymer.
Fig. 1 Optical microscopic images of Pickering HIPEs stabilized by lignin particles of different inter phase fractions: 75% (a), 80 (b), 85% (c). Scale
bars, 20 mm.
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Fig. 2 SEM images of the UFMF samples derived from HIPEs-1 to HIPEs-11, and the UFMF11 after regeneration (UFMF11-R).
To understand the wettability properties of the adsorbent, 7.15% for lignin and 10.52% for UFMF11) was mainly due to
water contact angle images are acquired as shown in Fig. 4. The desorption of physically absorbed water. This fact also suggests the
ꢀ
water contact angles of lignin and UFMF11 are about 76.41 and more hydrophilic surface of UFMF11. With the temperature
ꢀ
ꢀ
3
0.98 , respectively, directly suggesting the more hydrophilic increased to 800 C, the signicant loss of UFMF11 (74.56%) is
nature of UFMF11 than unmodied lignin. TGA graphs of lignin found for the thermal decomposition of the polymers, while the
and UFMF11 are presented in Fig. 5. As seen in Fig. 5, within the lignin maintains the 38.99% of residual carbon by calcining
ꢀ
initial temperature range (<250 C), the weight loss (approximately process.
Effects of pH and temperature for the adsorption of PCs
Optimization of pH value and temperature for adsorption
medium plays an important effect in the adsorption studies.
The effects of pH (a) and temperature (b) on the removal
percentage (%) of 2,4,5-TCP, 2,4,6-TCP and 4-NP are shown in
Fig. 3 FT-IR spectra of lignin and UFMF11.
Fig. 4 Water contact angle measurements of lignin (a) and UFMF11 (b).
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Adsorption kinetics
To determine an optimum contact time between PCs onto
UFMF11, adsorption capacities of 2,4,5-TCP, 2,4,6-TCP and 4-
NP were measured as a function of contact time, and the results
are presented in Fig. 7. The adsorption capacity of 2,4,5-TCP
increased rapidly within 25 min, and the another 20 min of
t
contact time was required for the Q value to reach the plateau.
The required time to reach to adsorption equilibrium was less
than the thermal-responsive and magnetic molecularly
imprinted polymers (TMMIPs) TMMIPs, which was reported by
our group. This fact may be attributed to the good hydrophi-
licity and unique porous structure of the UFMF11 matrix and
39
the prominent binding ability of UFMF11 towards 2,4,5-TCP.
The kinetics curves of 2,4,6-TCP and 4-NP have the same trend
with 2,4,5-TCP, whereas the fastest adsorption rate of 2,4,5-TCP
is observed. This result suggests that the diffusion rate of 2,4,5-
TCP through the testing solution to the surface of UFMF11 is
Fig. 5 TGA graphs of lignin and UFMF11.
40
superior to 2,4,6-TCP and 4-NP. Moreover, the group type and
position effects both inuence the adsorption, and follow the
order: 2,4,5-position substitution of chloride > 2,4,6-position
substitution of chloride > 4-position substitution of nitro-group,
showing that the substitution with the more groups has the
higher adsorption affinity.
Fig. 6, respectively. As shown in Fig. 6a, it could be observed that
the removal percentage of 2,4,5-TCP and 2,4,6-TCP are very high
at pH # 6.0 (pH # 7.0 for 4-NP), indicating no noticeable charge
interaction between PCs and UFMF11. The pK values of 2,4,5-
a
In order to examine the controlling mechanism of adsorp-
tion process, the effect of contact time on PCs adsorption was
studied in single solutions, and the kinetic data obtained were
analyzed using pseudo-rst-order and pseudo-second-order rate
TCP, 2,4,6-TCP and 4-NP were 6.94, 6.15 and 7.15, respec-
35–37
tively.
As a result, their molecular forms at acidic pH (#6.0)
participate in the binding process via the intermolecular forces
between PCs and the UFMF11. With the further increasing of
solution pH, the degree of PCs ionization enhances, which
weakens the interactions between the UFMF11 and PCs, such as
hydrogen bonding. This result suggests that pH ¼ 6.0 for
investigating the competitive adsorption of three PCs onto
UFMF11 is preferred. As described in Fig. 6b, the removal
percentage of three PCs slightly increased with the increase of
adsorption temperature. It is because that increasing tempera-
ture decreased the solution viscosity, and more adsorptive
molecules went across the external boundary layer and were
41
equations. The pseudo-rst-order and pseudo-second-order
equations can be expressed by eqn (3) and (4), respectively.
ln(Q ꢁ Q ) ¼ ln Q ꢁ k t
(3)
(4)
e
t
e
1
t
1
k2Qe
ꢁ1
t
¼
þ
2
Q
t
Q
t
ꢁ1
where Q (mg g ) and Q (mg g ) are the adsorption capacity at
e
t
equilibrium and time t (min), respectively. Values of k (L
1
ꢁ
1
ꢁ1
ꢁ1
min ) and k
ln(Q ) versus t and t/Q
ꢁ Q
pseudo-second-order kinetic rate constants, the initial adsorp-
2
(g mg min ) are calculated from the plot of
38
captured by sites in the internal pores. From a practical
viewpoint, the adsorption of three PCs was carried out at room
e
t
t
versus t, respectively. Based on the
ꢀ
ꢁ
1
ꢁ1
temperature (25 C).
tion rate (h, mg g min ) and half equilibrium time (t1/2, min)
Fig. 6 Effects of pH (a) and temperature (b) for the adsorption of PCs.
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43
be the rate-limiting step in the adsorption process. Compar-
ison of the parameters of h and t1/2 in Table 2, the adsorption
rate also follow the order: 2,4,5-TCP > 2,4,6-TCP > 4-NP.
Furthermore, the intra-particle diffusion kinetic model
based on the equation proposed by Weber and Morris was
44
tested.
pffiffi
Q
t
¼ Kdi t þ C
i
(7)
ꢁ
0.5 ꢁ1
where Kdi is the rate parameter of stage i (mg min
g
), it can
1/2
t i
be calculated from the slope of the straight line of Q versus t . C
is the intercept of stage i, and it provides information of the
thickness of boundary layer. Fig. 7 (inset) displays the intra-
particle diffusion kinetics, and related diffusion model constants
and correlation coefficients are listed in Table 3. In Fig. 7 (inset),
whole diffusion kinetics process is described with three separate
regions. The rst step in diffusion model is the mass transfer of
PCs molecules from the bulk solution to the external surface of the
UFMF11 or instantaneous stage, which is driven by high initial
Fig. 7 Pseudo first-order kinetics, second-order kinetics, and intra-
particle diffusion kinetics (inset) for adsorption of 2,4,5-TCP, 2,4,6-
TCP and 4-NP onto UFMF11.
45
PCs compounds concentration difference. In the rst stage, the
slope of 2,4,5-TCP adsorption plot is greater than that of 2,4,6-TCP
and 4-NP plot, which indicates a faster initial adsorption rate of
were also calculated according to the following equations. And 2,4,5-TCP. This is identical with pseudo-second-order model.
the adsorption rate constants and linear regression values from Furthermore, the linear lines of the second and third stages are
42
46
two kinetic equations were summarized in Table 2.
nearly parallel to the X axis, and this deviation might be due to the
signicant difference in the mass transfer rate in the initial and
(5) nal stages of adsorption, which can be proved from the much
2
h ¼ k
2 e
Q
higher values of Kd1 than that of Kd2 and Kd3.
1
t
1=2 ¼ k
(6)
2
Q
e
Adsorption isotherms
For three PCs, the pseudo-second-order model was more The relation between the adsorption capacity of 2,4,5-TCP,
applicable for describing the adsorption data as attested by the 2,4,6-TCP and 4-NP on the as-obtained UFMF11 and the equi-
2
higher correlation of coefficient (R ), compared to the pseudo- librium concentration is shown in the adsorption isotherm

rst-order kinetics (Table 2). Meanwhile, the calculated Q
e
(Fig. 8a). It can be observed that the adsorption capacity of 2,4,5-
values (Qe,c) by pseudo-second-order equation are also close to TCP on the UFMF11 obviously ascends with the increase of
experimental data (Qe,e), indicating that chemical process could equilibrium concentration, which could be explained by the
Table 2 The adsorption rate constants and linear regression values from two rate equations
Pseudo-rst-order equation
Pseudo-second-order equation
a
ꢁ1
b
ꢁ1
c
ꢁ1
2
b
ꢁ1
d
ꢁ1
e
ꢁ1
ꢁ1
f
2
Tested PCs
Q
e,e (mg g
)
Q
e,c (mmol g
)
k
1
(min
)
R
Q
e,c (mmol g
)
k
2
(min
)
h (mg g min
)
t
1/2
R
2
2
4
,4,6-TCP
,4,5-TCP
-NP
74.65
87.75
12.71
4.267
4.242
2.035
0.0066
0.0064
0.0022
0.7273 74.63
0.6585 87.72
0.8515 12.53
0.00893
0.00823
0.01041
49.76
63.29
1.635
1.500 1.000
1.386 1.000
7.664 0.9985
a
ꢁ1
b
ꢁ1
c
ꢁ1
Q
e,e (mg g ) is the experimental value of Q
e
.
Q
e,c (mg g ) is the calculated value of Q
e
.
k
1
(L min ) is the rate constant of rst-order adsorption
d
ꢁ1
ꢁ1
which was obtained from the plot of ln(Q
from the plot of t/q versus t. h (mg g min ) is the initial adsorption rate. t1/2 (min) is the half equilibrium time.
t
e
ꢁ Q
t
) versus t.
k (g mg min ) is the rate constant of second-order adsorption which was obtained
2
e
ꢁ1
ꢁ1
f
Table 3 Intra-particle diffusion model constants and correlation coefficients for adsorption of 2,4,6-TCP, 2,4,5-TCP and 4-NP on UFMF11
K
d1
K
d2
K
d3
(mg min
ꢁ1
ꢁ0.5 ꢁ1
ꢁ0.5 ꢁ1
ꢁ0.5 ꢁ1
2
2
2
Tested PCs
C
0
(mg L
)
(mg min
g
)
(mg min
g
)
g
)
C
1
C
2
C
3
(R
1
)
(R
2
)
3
(R )
2
2
4
,4,6-TCP
,4,5-TCP
-NP
100
100
50
5.463
9.928
0.3475
0.0299
0.0933
0.0541
0.0414
0.0892
0.0756
48.17
42.02
9.283
72.98
85.68
10.72
73.28
86.41
10.51
0.9996
1.000
0.9584
0.9367
0.9436
0.9862
0.9993
0.9906
0.7895
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Fig. 8 Equilibrium adsorption isotherms of 2,4,5-TCP, 2,4,6-TCP and 4-NP onto UFMF11 (a), and the effect of initial concentration to the
removal percentage (b).
high driving force for mass transfer. On the contrary, with the 0.9945 and 0.9948 for 2,4,5-TCP, 2,4,6-TCP and 4-NP, respec-
2
increase of equilibrium concentration, the adsorption capacity tively), than to the Freundlich model (R ¼ 0.9487, 0.9761 and
of 4-NP on the UFMF11 slowly increase.
In order to further understand the adsorption mechanisms gesting the monolayer adsorption. For UFMF11, 2,4,5-TCP had
of metallic ions with LDTC, Langmuir and Freundlich adsorp- a much higher K (Langmuir constant) than both 2,4,6-TCP and
0.9662 for 2,4,5-TCP, 2,4,6-TCP and 4-NP, respectively), sug-
50
L
tion isotherms models were chosen to t the data. The appli- 4-NP, which veries the advantageous 2,4,5-TCP adsorption of
cability of the isotherm models to the adsorption behaviours UFMF11. The max adsorption capacity of 2,4,5-TCP, 2,4,6-TCP
2
ꢀ
was studied by judging the correlation coefficient (R ). The and 4-NP at 25 C on the as-synthesized UFMF11 are observed to
ꢁ
1
ꢁ1
ꢁ1
Langmuir and Freundlich isotherm models were listed in eqn be 116.3 mg g , 78.74 mg g , 29.41 mg g , respectively. The
47,48
(8) and (9), respectively.
max adsorption capacity of 2,4,5-TCP is larger than those of 4
alkyltrimethylammonium bromides (alkyl ¼ C12) modied
natural bentonite, acid-activated montmorillonite (AMt), and
pillared-montmorillonite (AlMt), for which their values are 87.2
mg g , 96.4 mg g and 62.6 mg g . This result may be
assigned to the porous structure of UFMF11.
L m e
K Q C
Q
e
¼ 1
(8)
(9)
þ K
L e
C
ꢁ
1
ꢁ1
ꢁ1
1
/n
Q
e
¼ K
F
C
e
51,52
The effect of
ꢁ
1
initial concentration to the removal percentage was also evalu-
ated (Fig. 8b). As shown in Fig. 8b, when the initial concentra-
where C is the equilibrium concentration of PCs (mg L ), Q is
e
e
ꢁ1
the equilibrium adsorption capacity (mg g ). Qm is the
maximum adsorption capacity of the adsorbent, and K repre-
(mg g ) is the adsorption equi-
librium constant and 1/n is a measure of the surface
ꢁ1
ꢁ1
tion increases from 10 mg L to 50 mg L , the removal
percentage of 4-NP reduce by 31.75%; when the initial
L
ꢁ1
sents the affinity constant. K
F
ꢁ
1
ꢁ1
concentration increases from 20 mg L to 100 mg L , the
removal percentage of 2,4,5-TCP and 2,4,6-TCP reduce by
49
heterogeneity and exchange capacity. The Langmuir and
Freundlich adsorption constants and the corresponding corre-
lation coefficients were listed in Table 4. The adsorption data
are found to t better to the Langmuir model (R ¼ 0.9984,
13.95% and 30.98%, respectively, which indicates that UFMF11
performs better in adsorbing 2,4,5-TCP.
2
Competitive adsorption
The competitive adsorption of 2,4,5-TCP towards multi PCs was
also investigated. The HPLC chromatograms of mixed solutions
of 2,4,5-TCP + 4-NP (a), 2,4,5-TCP + 2,4,6-TCP (b), and 2,4,5-TCP
Table 4 Adsorption of isotherm constants for 2,4,6-TCP, 2,4,5-TCP
and 4-NP
+
2,4,6-TCP + 4-NP (c) with and without adsorption by UFMF11
Tested PCs
Adsorption
are shown in Fig. 9, and the removal percentage and adsorption
capacity of 2,4,5-TCP towards multi PCs are listed in Table 5.
From the Fig. 9, the selectivity sequence of the studied PCs by
isotherm models
Constants
2,4,6-TCP
2,4,5-TCP
4-NP
2
Langmuir
equation
R
0.9945
78.74
0.1714
0.9761
18.55
0.9984
116.3
0.1894
0.9487
21.74
0.9948 UFMF11 can be given as 2,4,5-TCP > 2,4,6-TCP > 4-NP according
ꢁ
1
Q
m
(mg g
(L mg
)
29.41
to the order of reduced peak areas, conrming the strong
affinity of UFMF11 to bind 2,4,5-TCP. By calculation, the
removal percentage and adsorption capacity of 2,4,5-TCP in the
presence of multi PCs followed the order 2,4,5-TCP + 4-NP >
ꢁ1
K
R
K
L
2
)
0.0424
0.9662
1.857
Freundlich
equation
ꢁ1
F
(L mg
)
1/n
0.3767
0.5211
0.6541
2,4,5-TCP + 2,4,6-TCP > 2,4,5-TCP + 2,4,6-TCP + 4-NP, suggesting
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Table 5 Removal percentage and adsorption capacity of 2,4,5-TCP
a
towards multi PCs
Removal
Adsorption capacity
ꢁ1
Multi PCs
percentage (%) of 2,4,5-TCP (mg g
)
2
2
2
,4,5-TCP + 2,4,6-TCP
,4,5-TCP + 4-NP
,4,5-TCP + 2,4,6-TCP + 4-NP 61.89
69.44
73.39
86.66
87.39
70.42
a
Equation between the peak area (Y) and initial concentration of 2,4,5-
2
TCP (X): Y ¼ 12 360 ꢂ X ꢁ 108 949 (R ¼ 0.9981).
Fig. 10 Adsorption capacity of tested PCs by UFMF11 after 4 regen-
eration cycles.
cycles for PCs removal were conducted, and the desorption
agent (i.e. mixed solution of methanol and 0.1 M NaOH) was
used to disturbed the hydrogen bond. The adsorption capacity
aer four regeneration cycles is shown in Fig. 10. The average
ꢁ1
adsorption capacities during 4 cycles were 84.47 mg g , 70.29
ꢁ1
ꢁ1
mg g and 10.35 mg g , respectively. Moreover, SEM image of
the UFMF11 aer regeneration (UFMF11-R) is studied in Fig. 2l,
it can be found that UFMF11-R has no obviously change aer
four times of regeneration. Thus, UFMF11 possessed efficient
regeneration, and will be a potential adsorbent for PCs
treatment.
Fig. 9 HPLC chromatograms of mixed solutions of 2,4,5-TCP + 4-NP
(
a), 2,4,5-TCP + 2,4,6-TCP (b), and 2,4,5-TCP + 2,4,6-TCP + 4-NP (c)
with and without adsorption by UFMF11.
Conclusions
In this study, a simple and effective method based on lignin-
based Pickering HIPEs template was adopted to prepare the
UFMF with the porous matrix, hydrophilic surface, and high
content of binding groups. Then UFMF was used as a new
adsorbent to remove the three typical PCs (i.e. 2,4,5-TCP, 2,4,6-
TCP and 4-NP) via the batch mode procedure. The results
demonstrate that the adsorption rate, adsorption capacity, and
selectivity sequence all follow the order 2,4,5-TCP > 2,4,6-TCP >
that 2,4,6-TCP has the stronger competitive adsorption ability
than that of 4-NP owing to the similar structure with 2,4,5-TCP.
Furthermore, UFMF11 retains the excellent adsorption effect for
2,4,5-TCP under the condition of multi PCs, especially in the
presence of 2,4,6-TCP and 4-NP.
Regeneration ability
4-NP, which can be explained by group type and position effects,
The reusability of the as-prepared UFMF11 is very important and the fact that substitution with the more groups has the
to commercialization. Thus, the four adsorption–desorption higher adsorption affinity is described. Further more, UFMF11
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has the excellent affinity to bind 2,4,5-TCP, and 2,4,6-TCP has 17 A. Menner, R. Powell and A. Bismarck, Macromolecules, 2006,
the stronger competitive adsorption ability than that of 4-NP 39, 2034–2035.
owing to the similar structure with 2,4,5-TCP. All these 18 Y. Yang, Z. J. Wei, C. Y. Wang and Z. Tong, Chem. Commun.,
considerations suggested that lignin-based Pickering HIPEs 2013, 49, 7144–7146.
template could serve as a promising method to design the 19 V. O. Ikem, A. Menner and A. Bismarck, Langmuir, 2010, 26,
porous adsorbent, and the as-prepared UFMF11 provide a new
possible way for the disposal of PCs-contained waste water.
8836–8841.
20 S. Livshin and M. S. Silverstein, J. Polym. Sci., Part A: Polym.
Chem., 2009, 47, 4840–4845.
2
1 R. Ma, W. Y. Hao, X. L. Ma, Y. Tian and Y. D. Li, Angew.
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Acknowledgements
This work was nancially supported by the National Natural 22 A. L. Holmberg, N. A. Nguyen, M. G. Karavolias, K. H. Reno,
Science Foundation of China (no. 21574091 and no. 21576120),
Natural Science Foundation of Jiangsu Province (no.
R. P. Wool and T. H. Epps III, Macromolecules, 2016, 49,
1286–1295.
BK20131223, no. BK20160491, and no. BK20150433), National 23 M. Y. Gan, J. M. Pan, Y. L. Zhang, X. H. Dai, Y. J. Yin, Q. Qu
Postdoctoral Science Foundation (no. 2015M580398), Post- and Y. S. Yan, Chem. Eng. J., 2014, 257, 317–327.
doctoral Science Foundation of Jiangsu Province (no. 24 Z. Liu, Z. Y. Du, H. Song, C. Y. Wang, F. Subhan, W. Xing and
1501027B), and Six Talent Peaks Project in Jiangsu Province
Z. F. Yan, J. Colloid Interface Sci., 2014, 416, 124–132.
(JNHB-016).
25 X. F. Sun and Z. K. Chai, J. Chromatogr. A, 2002, 943, 209–218.
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