Accepted Manuscript Title: Adsorption of methylene blue onto poly(cyclotriphosphazene-co-4,4 -sulfonyldiphenol) nanotubes: kinetics, isotherm and thermodynamics analysis Author: Zhonghui Chen Jianan Zhang Jianwei Fu Minghuan Wang Xuzhe Wang Runping Han Qun Xu PII: DOI: Reference:
S0304-3894(14)00239-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.03.053 HAZMAT 15826
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
23-12-2013 12-3-2014 25-3-2014
Please cite this article as: Z. Chen, J. Zhang, J. Fu, M. Wang, X. Wang, R. Han, Q. Xu, Adsorption of methylene blue onto poly(cyclotriphosphazene-co-4,4 sulfonyldiphenol) nanotubes: kinetics, isotherm and thermodynamics analysis, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.03.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Adsorption of methylene blue onto poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) nanotubes: kinetics, isotherm and thermodynamics analysis
Runping Han b, Qun Xu a,*
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou,
cr
a
ip t
Zhonghui Chen a, Jianan Zhang a, Jianwei Fu a,*, Minghuan Wang a, Xuzhe Wang a,
450052, P R China
School of Chemistry and Molecular Engineering, Zhengzhou University,
us
b
Ac ce p
te
d
M
an
Zhengzhou, 450052, P R China
∗
Corresponding authors. Address: School of Materials Science and Engineering,
Zhengzhou University, 75 Daxue Road, Zhengzhou, 450052, P R China. Tel.: 86-371-67767827; fax: 86-371-67767827 (J.W. Fu) Email address: [email protected] (J.W. Fu), [email protected] (Q. Xu)
1
Page 1 of 35
Abstract Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanotubes, an excellent adsorbent, were successfully synthesized by an in situ template method and used for the removal of methylene blue (MB) from aqueous solution. The morphology and
microscopy,
transmission
electron
microscope,
Fourier
ip t
structures of as-synthesized PZS nanotubes were characterized by scanning electron transform
infrared
cr
spectroscopy and N2 adsorption/desorption isotherms. The effects of temperature,
concentration, pH and contact time on MB adsorption were studied. It was favorable
us
for adsorption under the condition of basic and high temperature. The pseudo-first-order, pseudo-second-order and intraparticle diffusion models were used
an
to fit adsorption data in the kinetic studies. And results showed that the adsorption kinetics were more accurately described by the pseudo-second-order model. The
M
equilibrium isotherms were conducted using Freundlich and Langmuir models. It has been demonstrated that the better agreement was Langmuir isotherm with correlation
d
coefficient of 0.9933, equilibrium absorption capacity of 69.16 mg/g and the corresponding contact time of 15 min. Thermodynamic analyses showed that MB
te
adsorption onto the PZS nanotubes was endothermic and spontaneous and it was also
Ac ce p
a physisorption process.
Keywords: Methylene blue; Nanotubes; Kinetic model; Equilibrium isotherm; Thermodynamic
1. Introduction
Dyes are colour organic compounds and extensively used in the textile, leather,
paper, food, cosmetic and other industries [1]. They usually present in the effluents of these industries. Without reasonably processing, they can cause serious environment pollution since they are toxic to microorganisms and can impede the photosynthesis of aqueous flora [2]. Even worse, most of the organic dyes are harmful to human being due to their potential mutagenic and carcinogenic effects [3,4]. Therefore, the removal of dyes from wastewater has been seriously concerned. Various techniques including adsorption, biological treatment, chemical oxidation, 2
Page 2 of 35
coagulation/flocculation, membrane separation and ion exchange have been studied and applied [5-10]. However, owing to the aromatic groups and complex chemical structures of most dyes, they are hard to breakdown by biological and chemical treatments [11]. Besides, biological and chemical treatments may produce small
ip t
amount of toxic and carcinogenic products. To our delight, adsorption is a procedure
of choice for effluent treatment due to its efficiency, low cost and ease of operation. A
cr
wide range of adsorbent materials such as activation carbon, zeolite, silica and natural polymeric materials [12-15], etc have been applied. However, these conventional
efficiency [16,17] and long adsorption time [18].
us
adsorbents often display many defects and disadvantages such as low adsorption
an
Recently, large numbers of researchers have paid attention to novel adsorbents such as nanomaterials and synthetic polymer materials. Nanoparticles can be used as an
M
efficient adsorbent due to their small diffusion resistance and large specific surface area which can benefit the contact between adsorbents and adsorbates [19]. For
d
instance, Yao et al. explored the adsorption of methylene blue on carbon nanotubes [1]. Others also reported that hollow mesoporous carbon spheres [20], magnetic
te
multi-walled carbon nanotubes [21], halloysite nanotubes [11] and titanate nanotubes
Ac ce p
[22] could be used as efficient adsorbents for the removal of dyes from wastewater. Synthetic polymer materials have also been employed as adsorbents since it is possible to design their surface morphology, functional groups and internal structures [23,24]. It has been reported that polyaniline nanotubes base/silica composite [25], polypyrrole/TiO2 [26], polyurethane foam [27], porous polyurea [28] et al. were used
as excellent adsorbents for dye removal. Therefore, investigation on polymer-based nanomaterials which are applied as adsorbents should be paid increasing attention, since they can acquire the advantages of these two kinds of materials. Polymer nanotubes, a novel polymer-based nanomaterial, are an attractive alternative for the removal of dye contaminants from aqueous effluents because they have a large specific surface area, small size and tubular structure. However, up to the best of our knowledge, there are few papers in the literature [29], reporting the use of polymer nanotubes for dye removal from aqueous effluents. Therefore, the use of 3
Page 3 of 35
polymer nanotubes for dye adsorption requires new studies on this topic. In the present work, poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanotubes were demonstrated to be a high-efficiency adsorbent for the removal of methylene blue as a model compound for basic dyes from aqueous solution. PZS nanotubes were
ip t
facilely synthesized under mild conditions by precipitation polymerization between hexachlorocyclotriphosphazene (HCCP) and 4,4′-sulfonyldiphenol (BPS) [30,31].
cr
The adsorption of MB onto the PZS nanotubes was systematically investigated. For
instance, we studied the effects of contact time, temperature, initial concentration of
us
MB solution and initial solution pH on MB adsorption. The equilibrium, kinetics and thermodynamics of adsorption process were thoroughly studied. The saturated
an
adsorption capacity of the PZS nanotubes (69.16 mg/g) at 298 K was larger than carbon nanotubes (35.4 mg/g) [1], magnetite loaded multi-wall carbon nanotubes
M
(48.06 mg/g) [16], zeolite (10.86 mg/g) [17], polyurethane foam [23], Polyaniline nanotubes base/silica composite [25]，polyaniline nanotubes [29], etc. Besides, the
d
contact time (15 min), which was required to reach the equilibrium, was shorter than
te
the vast majority of adsorbents implying the procedure of MB adsorption onto the PZS nanotubes was highly rapid. The values of thermodynamic parameters (ΔG0, ΔH0
Ac ce p
and ΔS0) suggested the adsorption process was a spontaneous and endothermic
process.
2. Experimental 2.1 Materials
Hexachlorocyclotriphosphazene
(HCCP),
4,4′-sulfonyldiphenol
(BPS),
Triethylamine (TEA), tetrahydrofuran (THF), acetone and methylene blue (MB) were purchased from Sinaopharm Chemical Reagent Co., Ltd. All chemical reagents used in the experiment were of analytical reagent grade. HCCP was recrystallized from dry hexane followed by sublimation (60 112.5-113
, 0.05 mmHg) twice before use (mp =
). THF was redistilled for 2 h before use. The chemical formula of MB
was C16H18ClN3S. 3H2O and molecular weight was 373.9. All MB solutions were 4
Page 4 of 35
prepared by dissolving the required amounts of MB in deionized water. 2.2 Preparation of the PZS nanotubes The PZS nanotubes were prepared according to the modified method developed by our group [31,32]. In brief, 0.4 g HCCP and 1.296 g BPS were added into 100 mL
ip t
THF solvent. Before ultrasonic dispersion, a plastic board was placed in the bottom of
ultrasonic cleaner to reduce disturbance. Then, 1.5 mL TEA was added into the above under ultrasonic irradiation (50
cr
mixed solution. The mixed solution was kept at 40
W, 40 kHz) for 11 h. After reaction, the solution was filtered and then the precipitates
us
were washed three times with acetone and deionized water, respectively. Eventually, the products were dried in a vacuum oven to yield PZS nanotubes.
an
2.3 Adsorption experiments
Batch adsorption experiments were conducted using 50 mL Erlenmeyer flasks with
M
addition of the PZS nanotubes and MB solution at a shaking speed of 150 rpm in triplicate. Kinetic experiments were performed by mixing 15 mg PZS nanotubes into for
d
20 mL MB solution with a known initial concentration (100 mg/L) at 25 ± 1
different time intervals (5-50 min). Then, the suspensions were centrifuged at 3500
te
rpm for ten minutes. The concentrations of MB left in supernatant solutions were
Ac ce p
determined using a UV-vis spectrophotometer. The amount of MB adsorbed per unit mass of PZS nanotubes (q) and adsorption ratio (R) were calculated according to
Eqs.(1) and (2), respectively: q = V(c0 - ct)/m
(1);
R = 100(c0 - ct)/c0
(2).
Where c0 and ct are the initial and final (after adsorption) concentration of MB
solution (mg/L), respectively. V is the volume of MB solution (L) and m represents the mass of PZS nanotubes.
To study the adsorption isotherms, 15 mg PZS nanotubes were added into 20 mL MB solutions of different initial concentrations (20-140 mg/L) under the condition of natural pH (close to 7) and 25 ± 1 the system (5-45
for 15 min. Through varying the temperature of
) while other conditions were the same as the kinetic experiments,
we studied the effects of temperature on the adsorption of MB onto the PZS nanotubes. The initial pH of MB solution was adjusted to values in the range of 2-10 5
Page 5 of 35
by dropwise adding 0.1 mol/L NaOH or 0.1 mol/L HCl solutions to study the effects of initial solution pH. The effects of initial MB concentration were investigated by changing concentrations of MB solutions (20, 40, 60, 80, 100, 120, 140 mg/L). 2.4 Characterization
ip t
The size, morphology and structures of the PZS nanotubes were characterized by scanning electron microscopy (SEM) with a JEOL JSM-7401F field-emission
cr
microscope at an acceleration voltage of 5.0 KV and transmission electron microscopy (TEM). The specific surface area was measured on an ASAP 2020
us
adsorption apparatus using the Brunauer-Emmett-Teller (BET) method. Besides, the average pore size and total pore volume were also determined. The Fourier transform
an
infrared (FTIR) spectroscopy was measured on a Perkin-Elmer Paragon 1000 Fourier transform spectrometer at room temperature. A pH meter (Mettler Toledo Co., Ltd.
M
Shanghai, China) was used for the pH measurements. Malvern Zetasizer Nano ZS90 was used to measure the zeta potential of the PZS nanotubes. The adsorption
d
experiments were conducted on a TQZ-312 platform constant shaking incubator which can regulate the temperature and oscillation rate accurately. A UV-vis
te
spectrophotometer (Electrical and Instrument Analysis Instrument Co., Ltd. Shanghai,
Ac ce p
752N) was employed to determine the concentrations of MB left in supernatant solutions refering to the standard curve of MB at the maximum wavelength (665nm) of MB dye.
3. Results and discussion
3.1 Characterization of the PZS nanotubes Fig. 1a and b show the SEM and TEM images of the PZS nanotubes, respectively, synthesized by the precipitation polymerization of HCCP with BPS based on an in situ template mechanism. It can be easily observed that the polymerization products were almost short nanorods with capsicum-like morphology and exhibited a narrow length distribution (Fig. 1a). Typically, most of the nanorods were 3-6 μm in length, 6
Page 6 of 35
and the outer diameters of the two end of each nanorod were about 80 nm and 200-400 nm, respectively. The TEM image in Fig. 1b shows that the nanorods own hollow tubular structures with inner diameters of about 20-50 nm. It should be noted that most of the nanotube ends are closed.
ip t
The FTIR spectrum of the PZS nanotubes, displayed in Fig. 2a, is used to
qualitatively determinate characteristic functional groups which make the adsorption
cr
possible. It is obvious that the bands at 3424 and 3100 cm-1 are ascribed to the
stretching vibration of -OH bands and the two intensive adsorption peaks at 1591 and
us
1489 cm-1 are assigned to C=C of aromatic rings. These hydroxyl groups can be deprotonated at neutral or basic solutions, which can promote MB adsorption onto the
an
PZS nanotubes through providing adsorption sites for interaction with the cationic groups of MB [2]. Besides, the π-π stacking interactions between PZS nanotubes and
M
MB can occur during the adsorption process due to the existence of aromatic rings on the adsorbent and adsorbate [16]. The characteristic adsorption peaks at 1295 cm-1 and 1154 cm-1 ( O=S=O stretching vibration of the sulfonydiphenol units), 1187 cm-1 (P=N
d
stretching vibration of the cyclotriphosphazene structure), 880 cm-1 (P-N stretching
te
vibration), 940 cm-1 (Ar-P-O stretching vibration) imply the PZS nanotubes are
Ac ce p
successfully synthesized. Fig. 2b illustrates the fabrication procedure and the highly cross-linked structures of the PZS nanotubes. Herein, the TEA·HCl nanocrystals as core template could be easily removed by water washing, indicating the shell layers of the PZS nanotubes might possess large numbers of pores, which could provide a convenient channel for mass transfer. In addition, the existence of numerous electron-rich N and O atoms owning lone pair electrons can favor adsorption as they can work as basic reactive centers [33]. Fig. 3a and b show the nitrogen adsorption–desorption isotherm and the pore size distribution curve of the PZS nanotubes obtained at 77 K. The BET surface area (18.20 m2/g) and total pore volume (0.037 cm3/g) of the PZS nanotubes are calculated by the standard Brunauer–Emmett–Teller (BET) method and Horvath–Kawazoe (HK) method. The relatively high specific surface area of the PZS nanotubes can benefit the contact between the adsorption sites and adsorbate molecules. The isotherm plot of 7
Page 7 of 35
the PZS nanotubes, shown in Fig. 3a, is type IV with a hysteresis loop in the relative pressure range of 0.8 to 1.0 implying the presence of pore structures in the PZS nanotubes. As shown in Fig. 3b, the pore size distribution curve shows that the pore diameters for the PZS nanotubes are mainly centered at 6-9 nm and 20-50 nm,
ip t
respectively. Based on the TEM image and BET analysis above, we think that the former was resulted from the pore structure of the shell layer on the PZS nanotubes,
cr
and the later was related with the inner diameter of the PZS nanotubes. These pore structures could provide more adsorption sites. Meanwhile, MB molecules could
us
easily pass through the pore structures and enter into the interior of PZS nanotubes, which is beneficial to improve the amount adsorbed of MB on the PZS nanotubes.
an
3.2 Effects of initial pH
Fig. 4a shows the effects of initial solution pH on MB adsorption onto the PZS
M
nanotubes. It is obvious that along with the increase of pH values in the range of 2 to 10, the adsorption capacities present a significant increasing trend from 43.57 mg/g to
d
79.88 mg/g, suggesting basic solutions benefit MB adsorption. We can conclude pH is an important factor affecting MB adsorption onto the PZS nanotubes. Others have
te
also reported that solution pH can influence aqueous chemistry of dye molecules,
Ac ce p
surface binding-sites and surface charges of the adsorbent [16,34]. The zeta potential of the PZS nanotubes is displayed in Fig. 4b. The surface charge assessed by point of zero charge (pHpzc) is the point where the zeta potential is 0 and the pHpzc of the PZS nanotubes is about 1.2. When the solution pH is lower than 1.2, the surface charge of the PZS nanotubes are positive, which has an electrostatic repulsion to the cationic dye (MB). However, the adsorption capacity at pH value which is lower than 2 can still reach about 40 mg/g. The reason may be attributed to the π-π stacking interactions between the adsorbent and adsorbate and the existence of numerous electron-rich N and O atoms. When the solution pH value is higher than 1.2, the electrostatic attraction occurs between the electronegative PZS nanotubes and MB. The adsorption capacity (79.88 mg/g) at pH value of 10 can also demonstrate it. 3.3 Effects of temperature The temperature effect is evaluated under the condition of pH (6.47) and contact 8
Page 8 of 35
time (15min), as shown in Fig. 5. The adsorption capacities increase with the increasing of system temperature in the range of 5 to 45
, suggesting MB adsorption
onto the PZS nanotubes is endothermic. The adsorption capacity at 45
, reaching
maximum adsorption capacity (90.47 mg/g), can demonstrate that the dye adsorption
ip t
is favored at higher temperatures and the PZS nanotubes can be used as an excellent adsorbent. The reason can be ascribed to that the mobility of MB molecules increase
cr
with temperature and more molecules can interact with the active sites at PZS
nanotubes. It has also been reported that the rate of diffusion of dye molecules
us
increases along with the increasing of temperature, owing to the decrease in the viscosity of the solution [35].
an
3.4 Effects of initial MB concentration
The effects of initial MB concentration on adsorption are shown in Fig. 6. It is
M
conducted with different initial concentrations of MB (20-140 mg/L) in the presence of 15 mg PZS nanotubes for 15 min at 25
. The adsorption capacities present a trend
d
of increasing significantly at low concentrations and then slowly, as shown in Fig. 6a. At lower concentrations almost all MB molecules can contact with the active sites on
te
the surface of the PZS nanotubes. Nevertheless, the adsorption sites will reach
Ac ce p
saturation at high concentrations. Fig. 6b shows the adsorption ratios decrease drastically along with the increase of initial MB concentration. When initial MB concentration is 20 mg/L, the removal can reach about 95 % for only 15 min, indicating that the PZS nanotubes are an effective adsorbent. Fig. 6c shows the photographs of MB solutions before (left) and after (right) adsorption by PZS nanotubes. Obviously, MB solution becomes almost clear after adsorption. It should be noted that the white PZS nanotube agglomerations were not pulverized into powder in this observation experiment to clearly observe the MB adsorption onto PZS nanotubes. 3.5 Kinetic analysis In order to determine the time to reach equilibrium, the effect of contact time on MB adsorption onto the PZS nanotubes is studied at 25 , as shown in Fig. 7a. The time to reach equilibrium is only 15 min. The adsorption is initially rapid, and then 9
Page 9 of 35
constant since most vacant surface sites are available for adsorption during the initial stage. Besides, owing to the repulsive forces between the dye molecules on the PZS nanotubes and the bulk phase, the remaining vacant surface sites are hard to occupy. Adsorption kinetic studies are thoroughly studied due to their importance on the
ip t
investigation of the adsorption rate and mechanism. The pseudo-first-order and
pseudo-second-order kinetic models are employed to fit experimental data obtained
cr
from kinetic experiments. These two models can be expressed in linear form as Eqs.(3) and (4), respectively [1].
t/qt = 1/k2qe2 + t/qe
(3);
(4).
us
In(qe - qt) = In(qe) - k1t
Where qt and qe (mg/g) are the amounts of MB adsorbed at any time (min) and
an
equilibrium, respectively. And k1 (min-1) and k2 (g mg-1 min-1) are the pseudo-first-order and pseudo-second-order model rate constant, respectively. Fig. 7b
M
and c show the linear plots of these two models and the correlation coefficients (R2), k1, k2 and calculated qe,cal values are all shown in Table 1. The qe,cal of the pseudo-first-order model is 18.56 mg/g which deviates from the experimental qe
d
(69.16 mg/g) largely and its R2 is only 0.7538. However, the qe,cal of
te
pseudo-second-order model (74.85 mg/g) is very close to the experimental qe. The R2
Ac ce p
of pseudo-second-order model (0.9991) also suggests that MB adsorption onto the PZS nanotubes is according to the pseudo-second-order kinetic model. Weber’s intraparticle diffusion model is used to further identify the steps of
adsorption process. It can be expressed as Eq.(5) as follows [16]. qt = kit1/2 + c
(5).
Fig. 7d shows the multilinear plots of intraparticle diffusion process of MB adsorption onto the PZS nanotubes, indicating two steps have taken place. The two steps correspond to film diffusion which is the diffusion of MB molecules from solution to the external surface of the PZS nanotubes, and intraparticle diffusion, namely the diffusion of MB molecules through pores of the PZS nanotubes. The plot in Fig. 7d does not pass through the origin, suggesting the intraparticle diffusion is not the rate-limiting step and some other mechanisms may also play an important role. The parameters and R2 of intraparticle diffusion model are shown in Table 1. It can be 10
Page 10 of 35
easily observed that ki1 is larger than ki2, indicating film diffusion is a rapid process while intraparticle diffusion is a gradual process. Besides, R12 (0.9984) and R22 (0.9101) can demonstrate good applicability of Weber’s intraparticle diffusion model in studying steps of MB adsorption onto the PZS nanotubes.
ip t
Based on the above analysis, we proposed the adsorption process and mechanism for MB on the PZS nanotube, as illustrated in Fig. 8a. Under the incubator shaking
cr
speed of 150 rpm, the MB molecules diffuse to the adsorbent rapidly in the film
diffusion stage due to the existence of driving forces which result from the initial dye
us
concentrations. When the cationic MB molecules are close to the external surface of the PZS nanotubes with a large number of negatively charged sites, the electrostatic
an
attraction and π-π stacking interactions will play a major role. However, the adsorption rate becomes very low and then constant in the final stage resulting from
M
the repulsive forces on account of large amounts of MB molecules on the PZS nanotubes. FTIR analysis was carried out to further gain insight into the adsorption
d
mechanism. Fig. 8b shows the FTIR spectra of the PZS nanotubes before and after the MB adsorption. Based on the aforementioned results, the surface of PZS nanotubes
te
has abundant hydroxyl groups confirming by the presence of the characteristic
Ac ce p
absorption peak at 3424 cm-1. After adsorption of MB, the peak at 3424 cm-1 appeared an obvious red shift (3412 cm-1), which reveals that the hydroxyl groups may play an
important role for MB adsorption onto the PZS nanotubes. MB is a kind of cationic dye which can be adsorbed easily by electrostatic forces on negatively charged surfaces. Therefore, the red shift of hydroxyl group peak after adsorption may be associated to the electrostatic attraction between PZS nanotubes and MB. In addition, the adsorption peaks at 1593 cm-1, 1385 cm-1, and 1322 cm-1, being ascribed to the vibration of the aromatic ring, C-N bond, and CH3 group for MB respectively [16, 22], can be observed in the FTIR spectrum of the PZS nanotubes after the MB adsorption. This reflects the MB has been anchored on the surface of PZS nanotubes during the adsorption. It should be noted that the peaks associated with the vibration of the aromatic ring and C-N bond seem to be a significant decrease in intensity, which might result from the π-π stacking interactions between the aromatic backbone of MB 11
Page 11 of 35
and the aromatic skeleton of PZS. Therefore, the electrostatic attraction and π-π stacking interactions between PZS and MB could be responsible for the high adsorption ability of the PZS nanotubes. In addition, the large BET surface area and porous structure of the PZS nanotubes are also of great benefit to improve the amount
ip t
adsorbed of MB on the PZS nanotubes. 3.6 Adsorption isotherms
cr
As equilibrium adsorption isotherm is important to the design of adsorption systems and can provide information about the relationship between the concentration
us
of dye in solution and the amount of dye adsorbed on the solid phase when both phase are in equilibrium, the isotherm data are fitted to Freundlich and Langmuir models.
In qe = In KF + 1/n In Ce
an
These two models are expressed in linear form as Eq.(6) and Eq.(7), respectively. (6);
Ce/qe = 1/q0 KL + 1/q0 Ce
(7).
M
Where KF and n are Freundlich constants. KL and q0 are the Langmuir constants related to adsorption rate and adsorption capacity. Fig. 9a and b show Freundlich and , respectively.
d
Langmuir isotherms for MB adsorption onto the PZS nanotubes at 25
The relative parameters are shown in Table 2. Values of n in Freundlich model can
te
give an indication of how favorable the adsorption process. It has been reported that
Ac ce p
values of n in the range of 2-10 represent good, 1-2 moderately difficult, and less than 1 poor adsorption characteristics [1]. The n (4.555) of MB adsorption onto the PZS nanotubes indicates the PZS nanotubes are extremely suitable to adsorb MB dye. However, the correlation coefficients (R2) of Freundlich isotherm (0.9192) is smaller
than Langmuir isotherm (0.9933), implying the adsorption of MB onto PZS nanotubes follows the Langmuir isotherm. Moreover, the monolayer adsorption capacity determined from the Langmuir isotherm is 72.83 mg/g which approaches experimental data (69.16 mg/g). Table 3 displays the saturated adsorption capacities for MB on the PZS nanotubes and other adsorbents at the condition of 298 K and pH which is close to 7. It is easy to conclude that the PZS nanotubes can be employed as an excellent adsorbent. RL, a dimensionless constant separation factor can be used to evaluate the feasibility of adsorption on adsorbent and determines from Eq.(8) [36]. RL = 1/(1 + bC0)
(8). 12
Page 12 of 35
Where b is Langmuir constant (KL) and C0 is initial dye concentration. The value of RL indicates the type of the isotherm to be either irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), unfavorable (RL > 1). The calculated RL values are in the range of 0.028-0.17, indicating MB adsorption onto the PZS nanotubes is favorable.
ip t
3.7 Thermodynamic analyses
The inherent energetic changes related to the adsorption process can be reflected by
cr
thermodynamic parameters, such as free energy change (ΔG0, kJmol-1), entropy
using Langmuir isotherm by the following equations: ΔG0 = - RT In(KL)
us
change (ΔS0, Jmol-1K-1), and enthalpy change (ΔH0, kJmol-1). They can be determined
In(KL) = ΔS0/R - ΔH0/RT
(9);
(10).
an
Where KL is the Langmuir equilibrium constant (l/mol); R and T represent the universal gas constant (8.314JK-1mol-1) and the system temperature (K), respectively.
M
ΔG0 can be easily determined from Eq.(9). ΔS0 and ΔH0 are determined from the intercept and slope of the van’t Hoff plots of In(KL) versus 1/T.
negative
at
298.15
K,
d
The ΔG0 (-10.68 KJ/mol) of the adsorption of MB onto the PZS nanotubes is confirming
this
adsorption
is
spontaneous
and
te
thermodynamically favorable. Besides, it has been demonstrated that ΔG0 is less than
Ac ce p
0 drastically suggesting that the driving force of adsorption is greater and the adsorption capacity is higher [35]. The values of ΔS0 and ΔH0 are 66.81 Jmol-1K-1 and
9.84 kJ/mol, respectively. Researchers have reported that the ΔH0 of physisorption is
smaller than 40 KJ/mol [37], implying MB adsorption onto the PZS nanotubes is a physisorption process. And the value of ΔH0 is positive, suggesting the adsorption reaction is endothermic. It is also demonstrated by the temperature effect mentioned above that adsorption capacities increase with the increasing of system temperature. The positive ΔS0 means the degrees of freedom increase at the adsorbent-adsorbate
interface during the adsorption process. All the thermodynamic parameters (ΔG0, ΔH0 and ΔS0) indicate that MB adsorption onto the PZS nanotubes is favorable and PZS nanotubes can be used as an excellent adsorbent to remove MB from aqueous solution.
13
Page 13 of 35
4. Conclusions In this study, the PZS nanotubes were successfully prepared by a facile precipitation polymerization method under mild conditions. The as-synthesized PZS nanotubes which own hollow tubular structures, relatively high specific surface area,
ip t
pore structures, numerous hydroxyl groups, aromatic rings and electron-rich N and O
atoms have been proved to be an excellent absorbent for the removal of MB from could reach up to
cr
aqueous solution. The adsorption capacity at equilibrium at 25
69.16 mg/g and the corresponding contact time was only 15 min which was shorter
us
than the vast majority of adsorbents. Results also showed that MB adsorption onto the PZS nanotubes was highly dependent on temperature, concentration and pH of MB In
the
kinetic
studies,
the
pseudo-first-order
an
solution.
kinetic
model,
pseudo-second-order kinetic model and intraparticle diffusion model were used to fit
M
adsorption data. The pseudo-second-order kinetic model could better describe adsorption kinetics and intraparticle diffusion model also demonstrated the
d
intraparticle diffusion was not the rate-limiting step. The R2 (0.9933) of the Langmuir isotherm and its monolayer adsorption capacity (72.83mg/g) which is highly
te
approaching experimental data (69.16 mg/g), indicated that the adsorption of MB onto
Ac ce p
the PZS nanotubes followed the Langmuir isotherm. The values of thermodynamic parameters (ΔG0, ΔH0 and ΔS0) suggested that MB adsorption onto the PZS nanotubes
was endothermic and spontaneous. Besides, it was a physisorption process. It is expected that this study might develop a novel and efficient adsorbent based on PZS nanotubes for removal of dyes from aqueous solution, and other polymer-based nanomaterials could also been applied.
Acknowledgments We are grateful to the National Natural Science Foundation of China (No. 51003098, 51173170), the Foundation of Henan Educational Committee for Key Program of Science and Technology (No. 12A430014, 14B430036), and the financial support from the Program for New Century Excellent Talents in Universities (NCET).
14
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[35] C.H. Wu, Adsorption of reactive dye onto carbon nanotubes: Equilibrium, kinetics and thermodynamics, J. Hazard. Mater. 144 (2007) 93–100. [36] T.W. Weber, R.K. Chakravorti, Pore and solid diffusion models for fixed-bed adsorbers, AlChE J. 20 (1974) 228–238. [37] M. Kara, H. Yuzer, E. Sabah, M.S. Celik, Adsorption of cobalt from aqueous
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an
us
cr
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solutions onto sepiolite, Water Res. 37 (2003) 224–232.
19
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Tables
Model parameters
R2
Pseudo-first order
qe,cal = 18.56 mg/g k1 = 0.0755 min-1
0.7538
Pseudo-second order
qe,cal = 74.85 mg/g k2 = 0.0067 g mg-1 min-1
0.9991
Intraparticle diffusion
ki1 = 11.41 mg/g min1/2 c1 = 24.82 mg/g ki2 = 0.8634 mg/g min1/2 c2 = 65.54 mg/g
us
cr
Models
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Table 1. Kinetic models for MB adsorption onto the PZS nanotubes.
0.9984
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te
d
M
an
0.9101
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Parameters
R2
Freundlich
kF = 28.0 L/mg n = 4.555
0.9192
Langmuir
q0 = 72.83 mg/g kL = 0.2444 L/mg
0.9933
Ac ce p
te
d
M
an
us
cr
Isotherms
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Table 2 Isotherm parameters for MB adsorption onto the PZS nanotubes.
21
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Table 3 Comparison of saturated adsorption capacities for MB on different adsorbents at the condition of 298 K and pH which is close to 7. q (mg/g)
References
Carbon nanotubes
35.4
[1] [16]
Polyurethane foam
[23]
us
[17]
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Magnetite loaded multi-wall 48.06 carbon nanotubes Zeolite 10.86
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Adsorbents
23.03
[25]
4.8
[29]
PZS nanotubes
69.16
This work
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te
d
M
Polyaniline nanotubes
an
Polyaniline nanotubes base/ 5.38 silica composite
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Captions to Figures Fig. 1. SEM (a) and TEM (b) images of the as-prepared PZS nanotubes.
Fig. 2. (a) FTIR spectrum of the PZS nanotubes. (b) Schematic illustration of the
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preparation of the PZS nanotubes (top). The polycondensation of comonomers HCCP
us
nanotubes (bottom). (P3N3) indicates other phosphazene cores.
cr
and BPS and the highly cross-linked chemical structure of the as-synthesized PZS
Fig. 3. (a) Nitrogen adsorption–desorption isotherm obtained at 77 K for the PZS
an
nanotubes. (b) The pore size distribution curve of the PZS nanotubes.
Fig. 4. (a) Effects of initial pH on the adsorption of MB onto the PZS nanotubes.
M
Conditions: initial MB concentration: 100 mg/L; mass of adsorbent: 15 mg; temperature: 298 K; contact time: 15 min. (b) Zeta potential of the PZS nanotubes at
te
d
different pH values.
Fig. 5. Effects of temperature on the adsorption of MB onto the PZS nanotubes.
Ac ce p
Conditions: initial MB concentration: 100 mg/L; mass of adsorbent: 15 mg; pH: 6.47; contact time: 15 min.
Fig. 6. (a) Effects of initial MB concentration on the adsorption capacity. (b) Effects of initial MB concentration on the adsorption ratio. Conditions: mass of adsorbent: 15 mg; pH: 6.47; temperature: 298 K; contact time: 15 min. (c) The photographs of MB solutions before (left) and after (right) adsorption by the PZS nanotubes.
Fig. 7. (a) Effects of contact time on the adsorption of MB onto the PZS nanotubes. (b) Pseudo-first-order model, (c) Pseudo-second-order model and (d) intraparticle diffusion model for the adsorption of MB onto the PZS nanotubes.
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Fig. 8. (a) Schematic illustration of the adsorption process and mechanism for MB on the PZS nanotubes. (b) FTIR spectra of the PZS nanotubes before and after the MB adsorption, and pure MB.
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Fig. 9. Freundlich (a) and Langmuir (b) isotherms for the adsorption of MB onto the
Ac ce p
te
d
M
an
us
cr
PZS nanotubes.
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Graphical Abstract (for review)
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ed
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Graphical abstract
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*Highlights (for review)
Research Highlights 1. Polyphosphazene nanotube as an adsorbent could be facilely synthesized. 2. The adsorbent owns numerous electron-rich N and P atoms and hydroxyl groups.
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3. The adsorbent was an efficient and specific adsorbent for the removal of MB. 4. The pseudo-second-order model could be better to describe the adsorption of MB.
Ac
ce pt
ed
M
an
us
cr
5. The MB adsorption onto PZS nanotubes was endothermic and spontaneous.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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S0304-3894(14)00239-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.03.053 HAZMAT 15826
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
23-12-2013 12-3-2014 25-3-2014
Please cite this article as: Z. Chen, J. Zhang, J. Fu, M. Wang, X. Wang, R. Han, Q. Xu, Adsorption of methylene blue onto poly(cyclotriphosphazene-co-4,4 sulfonyldiphenol) nanotubes: kinetics, isotherm and thermodynamics analysis, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.03.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Adsorption of methylene blue onto poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) nanotubes: kinetics, isotherm and thermodynamics analysis
Runping Han b, Qun Xu a,*
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou,
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a
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Zhonghui Chen a, Jianan Zhang a, Jianwei Fu a,*, Minghuan Wang a, Xuzhe Wang a,
450052, P R China
School of Chemistry and Molecular Engineering, Zhengzhou University,
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b
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d
M
an
Zhengzhou, 450052, P R China
∗
Corresponding authors. Address: School of Materials Science and Engineering,
Zhengzhou University, 75 Daxue Road, Zhengzhou, 450052, P R China. Tel.: 86-371-67767827; fax: 86-371-67767827 (J.W. Fu) Email address: [email protected] (J.W. Fu), [email protected] (Q. Xu)
1
Page 1 of 35
Abstract Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanotubes, an excellent adsorbent, were successfully synthesized by an in situ template method and used for the removal of methylene blue (MB) from aqueous solution. The morphology and
microscopy,
transmission
electron
microscope,
Fourier
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structures of as-synthesized PZS nanotubes were characterized by scanning electron transform
infrared
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spectroscopy and N2 adsorption/desorption isotherms. The effects of temperature,
concentration, pH and contact time on MB adsorption were studied. It was favorable
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for adsorption under the condition of basic and high temperature. The pseudo-first-order, pseudo-second-order and intraparticle diffusion models were used
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to fit adsorption data in the kinetic studies. And results showed that the adsorption kinetics were more accurately described by the pseudo-second-order model. The
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equilibrium isotherms were conducted using Freundlich and Langmuir models. It has been demonstrated that the better agreement was Langmuir isotherm with correlation
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coefficient of 0.9933, equilibrium absorption capacity of 69.16 mg/g and the corresponding contact time of 15 min. Thermodynamic analyses showed that MB
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adsorption onto the PZS nanotubes was endothermic and spontaneous and it was also
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a physisorption process.
Keywords: Methylene blue; Nanotubes; Kinetic model; Equilibrium isotherm; Thermodynamic
1. Introduction
Dyes are colour organic compounds and extensively used in the textile, leather,
paper, food, cosmetic and other industries [1]. They usually present in the effluents of these industries. Without reasonably processing, they can cause serious environment pollution since they are toxic to microorganisms and can impede the photosynthesis of aqueous flora [2]. Even worse, most of the organic dyes are harmful to human being due to their potential mutagenic and carcinogenic effects [3,4]. Therefore, the removal of dyes from wastewater has been seriously concerned. Various techniques including adsorption, biological treatment, chemical oxidation, 2
Page 2 of 35
coagulation/flocculation, membrane separation and ion exchange have been studied and applied [5-10]. However, owing to the aromatic groups and complex chemical structures of most dyes, they are hard to breakdown by biological and chemical treatments [11]. Besides, biological and chemical treatments may produce small
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amount of toxic and carcinogenic products. To our delight, adsorption is a procedure
of choice for effluent treatment due to its efficiency, low cost and ease of operation. A
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wide range of adsorbent materials such as activation carbon, zeolite, silica and natural polymeric materials [12-15], etc have been applied. However, these conventional
efficiency [16,17] and long adsorption time [18].
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adsorbents often display many defects and disadvantages such as low adsorption
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Recently, large numbers of researchers have paid attention to novel adsorbents such as nanomaterials and synthetic polymer materials. Nanoparticles can be used as an
M
efficient adsorbent due to their small diffusion resistance and large specific surface area which can benefit the contact between adsorbents and adsorbates [19]. For
d
instance, Yao et al. explored the adsorption of methylene blue on carbon nanotubes [1]. Others also reported that hollow mesoporous carbon spheres [20], magnetic
te
multi-walled carbon nanotubes [21], halloysite nanotubes [11] and titanate nanotubes
Ac ce p
[22] could be used as efficient adsorbents for the removal of dyes from wastewater. Synthetic polymer materials have also been employed as adsorbents since it is possible to design their surface morphology, functional groups and internal structures [23,24]. It has been reported that polyaniline nanotubes base/silica composite [25], polypyrrole/TiO2 [26], polyurethane foam [27], porous polyurea [28] et al. were used
as excellent adsorbents for dye removal. Therefore, investigation on polymer-based nanomaterials which are applied as adsorbents should be paid increasing attention, since they can acquire the advantages of these two kinds of materials. Polymer nanotubes, a novel polymer-based nanomaterial, are an attractive alternative for the removal of dye contaminants from aqueous effluents because they have a large specific surface area, small size and tubular structure. However, up to the best of our knowledge, there are few papers in the literature [29], reporting the use of polymer nanotubes for dye removal from aqueous effluents. Therefore, the use of 3
Page 3 of 35
polymer nanotubes for dye adsorption requires new studies on this topic. In the present work, poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanotubes were demonstrated to be a high-efficiency adsorbent for the removal of methylene blue as a model compound for basic dyes from aqueous solution. PZS nanotubes were
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facilely synthesized under mild conditions by precipitation polymerization between hexachlorocyclotriphosphazene (HCCP) and 4,4′-sulfonyldiphenol (BPS) [30,31].
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The adsorption of MB onto the PZS nanotubes was systematically investigated. For
instance, we studied the effects of contact time, temperature, initial concentration of
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MB solution and initial solution pH on MB adsorption. The equilibrium, kinetics and thermodynamics of adsorption process were thoroughly studied. The saturated
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adsorption capacity of the PZS nanotubes (69.16 mg/g) at 298 K was larger than carbon nanotubes (35.4 mg/g) [1], magnetite loaded multi-wall carbon nanotubes
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(48.06 mg/g) [16], zeolite (10.86 mg/g) [17], polyurethane foam [23], Polyaniline nanotubes base/silica composite [25]，polyaniline nanotubes [29], etc. Besides, the
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contact time (15 min), which was required to reach the equilibrium, was shorter than
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the vast majority of adsorbents implying the procedure of MB adsorption onto the PZS nanotubes was highly rapid. The values of thermodynamic parameters (ΔG0, ΔH0
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and ΔS0) suggested the adsorption process was a spontaneous and endothermic
process.
2. Experimental 2.1 Materials
Hexachlorocyclotriphosphazene
(HCCP),
4,4′-sulfonyldiphenol
(BPS),
Triethylamine (TEA), tetrahydrofuran (THF), acetone and methylene blue (MB) were purchased from Sinaopharm Chemical Reagent Co., Ltd. All chemical reagents used in the experiment were of analytical reagent grade. HCCP was recrystallized from dry hexane followed by sublimation (60 112.5-113
, 0.05 mmHg) twice before use (mp =
). THF was redistilled for 2 h before use. The chemical formula of MB
was C16H18ClN3S. 3H2O and molecular weight was 373.9. All MB solutions were 4
Page 4 of 35
prepared by dissolving the required amounts of MB in deionized water. 2.2 Preparation of the PZS nanotubes The PZS nanotubes were prepared according to the modified method developed by our group [31,32]. In brief, 0.4 g HCCP and 1.296 g BPS were added into 100 mL
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THF solvent. Before ultrasonic dispersion, a plastic board was placed in the bottom of
ultrasonic cleaner to reduce disturbance. Then, 1.5 mL TEA was added into the above under ultrasonic irradiation (50
cr
mixed solution. The mixed solution was kept at 40
W, 40 kHz) for 11 h. After reaction, the solution was filtered and then the precipitates
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were washed three times with acetone and deionized water, respectively. Eventually, the products were dried in a vacuum oven to yield PZS nanotubes.
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2.3 Adsorption experiments
Batch adsorption experiments were conducted using 50 mL Erlenmeyer flasks with
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addition of the PZS nanotubes and MB solution at a shaking speed of 150 rpm in triplicate. Kinetic experiments were performed by mixing 15 mg PZS nanotubes into for
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20 mL MB solution with a known initial concentration (100 mg/L) at 25 ± 1
different time intervals (5-50 min). Then, the suspensions were centrifuged at 3500
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rpm for ten minutes. The concentrations of MB left in supernatant solutions were
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determined using a UV-vis spectrophotometer. The amount of MB adsorbed per unit mass of PZS nanotubes (q) and adsorption ratio (R) were calculated according to
Eqs.(1) and (2), respectively: q = V(c0 - ct)/m
(1);
R = 100(c0 - ct)/c0
(2).
Where c0 and ct are the initial and final (after adsorption) concentration of MB
solution (mg/L), respectively. V is the volume of MB solution (L) and m represents the mass of PZS nanotubes.
To study the adsorption isotherms, 15 mg PZS nanotubes were added into 20 mL MB solutions of different initial concentrations (20-140 mg/L) under the condition of natural pH (close to 7) and 25 ± 1 the system (5-45
for 15 min. Through varying the temperature of
) while other conditions were the same as the kinetic experiments,
we studied the effects of temperature on the adsorption of MB onto the PZS nanotubes. The initial pH of MB solution was adjusted to values in the range of 2-10 5
Page 5 of 35
by dropwise adding 0.1 mol/L NaOH or 0.1 mol/L HCl solutions to study the effects of initial solution pH. The effects of initial MB concentration were investigated by changing concentrations of MB solutions (20, 40, 60, 80, 100, 120, 140 mg/L). 2.4 Characterization
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The size, morphology and structures of the PZS nanotubes were characterized by scanning electron microscopy (SEM) with a JEOL JSM-7401F field-emission
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microscope at an acceleration voltage of 5.0 KV and transmission electron microscopy (TEM). The specific surface area was measured on an ASAP 2020
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adsorption apparatus using the Brunauer-Emmett-Teller (BET) method. Besides, the average pore size and total pore volume were also determined. The Fourier transform
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infrared (FTIR) spectroscopy was measured on a Perkin-Elmer Paragon 1000 Fourier transform spectrometer at room temperature. A pH meter (Mettler Toledo Co., Ltd.
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Shanghai, China) was used for the pH measurements. Malvern Zetasizer Nano ZS90 was used to measure the zeta potential of the PZS nanotubes. The adsorption
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experiments were conducted on a TQZ-312 platform constant shaking incubator which can regulate the temperature and oscillation rate accurately. A UV-vis
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spectrophotometer (Electrical and Instrument Analysis Instrument Co., Ltd. Shanghai,
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752N) was employed to determine the concentrations of MB left in supernatant solutions refering to the standard curve of MB at the maximum wavelength (665nm) of MB dye.
3. Results and discussion
3.1 Characterization of the PZS nanotubes Fig. 1a and b show the SEM and TEM images of the PZS nanotubes, respectively, synthesized by the precipitation polymerization of HCCP with BPS based on an in situ template mechanism. It can be easily observed that the polymerization products were almost short nanorods with capsicum-like morphology and exhibited a narrow length distribution (Fig. 1a). Typically, most of the nanorods were 3-6 μm in length, 6
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and the outer diameters of the two end of each nanorod were about 80 nm and 200-400 nm, respectively. The TEM image in Fig. 1b shows that the nanorods own hollow tubular structures with inner diameters of about 20-50 nm. It should be noted that most of the nanotube ends are closed.
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The FTIR spectrum of the PZS nanotubes, displayed in Fig. 2a, is used to
qualitatively determinate characteristic functional groups which make the adsorption
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possible. It is obvious that the bands at 3424 and 3100 cm-1 are ascribed to the
stretching vibration of -OH bands and the two intensive adsorption peaks at 1591 and
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1489 cm-1 are assigned to C=C of aromatic rings. These hydroxyl groups can be deprotonated at neutral or basic solutions, which can promote MB adsorption onto the
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PZS nanotubes through providing adsorption sites for interaction with the cationic groups of MB [2]. Besides, the π-π stacking interactions between PZS nanotubes and
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MB can occur during the adsorption process due to the existence of aromatic rings on the adsorbent and adsorbate [16]. The characteristic adsorption peaks at 1295 cm-1 and 1154 cm-1 ( O=S=O stretching vibration of the sulfonydiphenol units), 1187 cm-1 (P=N
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stretching vibration of the cyclotriphosphazene structure), 880 cm-1 (P-N stretching
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vibration), 940 cm-1 (Ar-P-O stretching vibration) imply the PZS nanotubes are
Ac ce p
successfully synthesized. Fig. 2b illustrates the fabrication procedure and the highly cross-linked structures of the PZS nanotubes. Herein, the TEA·HCl nanocrystals as core template could be easily removed by water washing, indicating the shell layers of the PZS nanotubes might possess large numbers of pores, which could provide a convenient channel for mass transfer. In addition, the existence of numerous electron-rich N and O atoms owning lone pair electrons can favor adsorption as they can work as basic reactive centers [33]. Fig. 3a and b show the nitrogen adsorption–desorption isotherm and the pore size distribution curve of the PZS nanotubes obtained at 77 K. The BET surface area (18.20 m2/g) and total pore volume (0.037 cm3/g) of the PZS nanotubes are calculated by the standard Brunauer–Emmett–Teller (BET) method and Horvath–Kawazoe (HK) method. The relatively high specific surface area of the PZS nanotubes can benefit the contact between the adsorption sites and adsorbate molecules. The isotherm plot of 7
Page 7 of 35
the PZS nanotubes, shown in Fig. 3a, is type IV with a hysteresis loop in the relative pressure range of 0.8 to 1.0 implying the presence of pore structures in the PZS nanotubes. As shown in Fig. 3b, the pore size distribution curve shows that the pore diameters for the PZS nanotubes are mainly centered at 6-9 nm and 20-50 nm,
ip t
respectively. Based on the TEM image and BET analysis above, we think that the former was resulted from the pore structure of the shell layer on the PZS nanotubes,
cr
and the later was related with the inner diameter of the PZS nanotubes. These pore structures could provide more adsorption sites. Meanwhile, MB molecules could
us
easily pass through the pore structures and enter into the interior of PZS nanotubes, which is beneficial to improve the amount adsorbed of MB on the PZS nanotubes.
an
3.2 Effects of initial pH
Fig. 4a shows the effects of initial solution pH on MB adsorption onto the PZS
M
nanotubes. It is obvious that along with the increase of pH values in the range of 2 to 10, the adsorption capacities present a significant increasing trend from 43.57 mg/g to
d
79.88 mg/g, suggesting basic solutions benefit MB adsorption. We can conclude pH is an important factor affecting MB adsorption onto the PZS nanotubes. Others have
te
also reported that solution pH can influence aqueous chemistry of dye molecules,
Ac ce p
surface binding-sites and surface charges of the adsorbent [16,34]. The zeta potential of the PZS nanotubes is displayed in Fig. 4b. The surface charge assessed by point of zero charge (pHpzc) is the point where the zeta potential is 0 and the pHpzc of the PZS nanotubes is about 1.2. When the solution pH is lower than 1.2, the surface charge of the PZS nanotubes are positive, which has an electrostatic repulsion to the cationic dye (MB). However, the adsorption capacity at pH value which is lower than 2 can still reach about 40 mg/g. The reason may be attributed to the π-π stacking interactions between the adsorbent and adsorbate and the existence of numerous electron-rich N and O atoms. When the solution pH value is higher than 1.2, the electrostatic attraction occurs between the electronegative PZS nanotubes and MB. The adsorption capacity (79.88 mg/g) at pH value of 10 can also demonstrate it. 3.3 Effects of temperature The temperature effect is evaluated under the condition of pH (6.47) and contact 8
Page 8 of 35
time (15min), as shown in Fig. 5. The adsorption capacities increase with the increasing of system temperature in the range of 5 to 45
, suggesting MB adsorption
onto the PZS nanotubes is endothermic. The adsorption capacity at 45
, reaching
maximum adsorption capacity (90.47 mg/g), can demonstrate that the dye adsorption
ip t
is favored at higher temperatures and the PZS nanotubes can be used as an excellent adsorbent. The reason can be ascribed to that the mobility of MB molecules increase
cr
with temperature and more molecules can interact with the active sites at PZS
nanotubes. It has also been reported that the rate of diffusion of dye molecules
us
increases along with the increasing of temperature, owing to the decrease in the viscosity of the solution [35].
an
3.4 Effects of initial MB concentration
The effects of initial MB concentration on adsorption are shown in Fig. 6. It is
M
conducted with different initial concentrations of MB (20-140 mg/L) in the presence of 15 mg PZS nanotubes for 15 min at 25
. The adsorption capacities present a trend
d
of increasing significantly at low concentrations and then slowly, as shown in Fig. 6a. At lower concentrations almost all MB molecules can contact with the active sites on
te
the surface of the PZS nanotubes. Nevertheless, the adsorption sites will reach
Ac ce p
saturation at high concentrations. Fig. 6b shows the adsorption ratios decrease drastically along with the increase of initial MB concentration. When initial MB concentration is 20 mg/L, the removal can reach about 95 % for only 15 min, indicating that the PZS nanotubes are an effective adsorbent. Fig. 6c shows the photographs of MB solutions before (left) and after (right) adsorption by PZS nanotubes. Obviously, MB solution becomes almost clear after adsorption. It should be noted that the white PZS nanotube agglomerations were not pulverized into powder in this observation experiment to clearly observe the MB adsorption onto PZS nanotubes. 3.5 Kinetic analysis In order to determine the time to reach equilibrium, the effect of contact time on MB adsorption onto the PZS nanotubes is studied at 25 , as shown in Fig. 7a. The time to reach equilibrium is only 15 min. The adsorption is initially rapid, and then 9
Page 9 of 35
constant since most vacant surface sites are available for adsorption during the initial stage. Besides, owing to the repulsive forces between the dye molecules on the PZS nanotubes and the bulk phase, the remaining vacant surface sites are hard to occupy. Adsorption kinetic studies are thoroughly studied due to their importance on the
ip t
investigation of the adsorption rate and mechanism. The pseudo-first-order and
pseudo-second-order kinetic models are employed to fit experimental data obtained
cr
from kinetic experiments. These two models can be expressed in linear form as Eqs.(3) and (4), respectively [1].
t/qt = 1/k2qe2 + t/qe
(3);
(4).
us
In(qe - qt) = In(qe) - k1t
Where qt and qe (mg/g) are the amounts of MB adsorbed at any time (min) and
an
equilibrium, respectively. And k1 (min-1) and k2 (g mg-1 min-1) are the pseudo-first-order and pseudo-second-order model rate constant, respectively. Fig. 7b
M
and c show the linear plots of these two models and the correlation coefficients (R2), k1, k2 and calculated qe,cal values are all shown in Table 1. The qe,cal of the pseudo-first-order model is 18.56 mg/g which deviates from the experimental qe
d
(69.16 mg/g) largely and its R2 is only 0.7538. However, the qe,cal of
te
pseudo-second-order model (74.85 mg/g) is very close to the experimental qe. The R2
Ac ce p
of pseudo-second-order model (0.9991) also suggests that MB adsorption onto the PZS nanotubes is according to the pseudo-second-order kinetic model. Weber’s intraparticle diffusion model is used to further identify the steps of
adsorption process. It can be expressed as Eq.(5) as follows [16]. qt = kit1/2 + c
(5).
Fig. 7d shows the multilinear plots of intraparticle diffusion process of MB adsorption onto the PZS nanotubes, indicating two steps have taken place. The two steps correspond to film diffusion which is the diffusion of MB molecules from solution to the external surface of the PZS nanotubes, and intraparticle diffusion, namely the diffusion of MB molecules through pores of the PZS nanotubes. The plot in Fig. 7d does not pass through the origin, suggesting the intraparticle diffusion is not the rate-limiting step and some other mechanisms may also play an important role. The parameters and R2 of intraparticle diffusion model are shown in Table 1. It can be 10
Page 10 of 35
easily observed that ki1 is larger than ki2, indicating film diffusion is a rapid process while intraparticle diffusion is a gradual process. Besides, R12 (0.9984) and R22 (0.9101) can demonstrate good applicability of Weber’s intraparticle diffusion model in studying steps of MB adsorption onto the PZS nanotubes.
ip t
Based on the above analysis, we proposed the adsorption process and mechanism for MB on the PZS nanotube, as illustrated in Fig. 8a. Under the incubator shaking
cr
speed of 150 rpm, the MB molecules diffuse to the adsorbent rapidly in the film
diffusion stage due to the existence of driving forces which result from the initial dye
us
concentrations. When the cationic MB molecules are close to the external surface of the PZS nanotubes with a large number of negatively charged sites, the electrostatic
an
attraction and π-π stacking interactions will play a major role. However, the adsorption rate becomes very low and then constant in the final stage resulting from
M
the repulsive forces on account of large amounts of MB molecules on the PZS nanotubes. FTIR analysis was carried out to further gain insight into the adsorption
d
mechanism. Fig. 8b shows the FTIR spectra of the PZS nanotubes before and after the MB adsorption. Based on the aforementioned results, the surface of PZS nanotubes
te
has abundant hydroxyl groups confirming by the presence of the characteristic
Ac ce p
absorption peak at 3424 cm-1. After adsorption of MB, the peak at 3424 cm-1 appeared an obvious red shift (3412 cm-1), which reveals that the hydroxyl groups may play an
important role for MB adsorption onto the PZS nanotubes. MB is a kind of cationic dye which can be adsorbed easily by electrostatic forces on negatively charged surfaces. Therefore, the red shift of hydroxyl group peak after adsorption may be associated to the electrostatic attraction between PZS nanotubes and MB. In addition, the adsorption peaks at 1593 cm-1, 1385 cm-1, and 1322 cm-1, being ascribed to the vibration of the aromatic ring, C-N bond, and CH3 group for MB respectively [16, 22], can be observed in the FTIR spectrum of the PZS nanotubes after the MB adsorption. This reflects the MB has been anchored on the surface of PZS nanotubes during the adsorption. It should be noted that the peaks associated with the vibration of the aromatic ring and C-N bond seem to be a significant decrease in intensity, which might result from the π-π stacking interactions between the aromatic backbone of MB 11
Page 11 of 35
and the aromatic skeleton of PZS. Therefore, the electrostatic attraction and π-π stacking interactions between PZS and MB could be responsible for the high adsorption ability of the PZS nanotubes. In addition, the large BET surface area and porous structure of the PZS nanotubes are also of great benefit to improve the amount
ip t
adsorbed of MB on the PZS nanotubes. 3.6 Adsorption isotherms
cr
As equilibrium adsorption isotherm is important to the design of adsorption systems and can provide information about the relationship between the concentration
us
of dye in solution and the amount of dye adsorbed on the solid phase when both phase are in equilibrium, the isotherm data are fitted to Freundlich and Langmuir models.
In qe = In KF + 1/n In Ce
an
These two models are expressed in linear form as Eq.(6) and Eq.(7), respectively. (6);
Ce/qe = 1/q0 KL + 1/q0 Ce
(7).
M
Where KF and n are Freundlich constants. KL and q0 are the Langmuir constants related to adsorption rate and adsorption capacity. Fig. 9a and b show Freundlich and , respectively.
d
Langmuir isotherms for MB adsorption onto the PZS nanotubes at 25
The relative parameters are shown in Table 2. Values of n in Freundlich model can
te
give an indication of how favorable the adsorption process. It has been reported that
Ac ce p
values of n in the range of 2-10 represent good, 1-2 moderately difficult, and less than 1 poor adsorption characteristics [1]. The n (4.555) of MB adsorption onto the PZS nanotubes indicates the PZS nanotubes are extremely suitable to adsorb MB dye. However, the correlation coefficients (R2) of Freundlich isotherm (0.9192) is smaller
than Langmuir isotherm (0.9933), implying the adsorption of MB onto PZS nanotubes follows the Langmuir isotherm. Moreover, the monolayer adsorption capacity determined from the Langmuir isotherm is 72.83 mg/g which approaches experimental data (69.16 mg/g). Table 3 displays the saturated adsorption capacities for MB on the PZS nanotubes and other adsorbents at the condition of 298 K and pH which is close to 7. It is easy to conclude that the PZS nanotubes can be employed as an excellent adsorbent. RL, a dimensionless constant separation factor can be used to evaluate the feasibility of adsorption on adsorbent and determines from Eq.(8) [36]. RL = 1/(1 + bC0)
(8). 12
Page 12 of 35
Where b is Langmuir constant (KL) and C0 is initial dye concentration. The value of RL indicates the type of the isotherm to be either irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), unfavorable (RL > 1). The calculated RL values are in the range of 0.028-0.17, indicating MB adsorption onto the PZS nanotubes is favorable.
ip t
3.7 Thermodynamic analyses
The inherent energetic changes related to the adsorption process can be reflected by
cr
thermodynamic parameters, such as free energy change (ΔG0, kJmol-1), entropy
using Langmuir isotherm by the following equations: ΔG0 = - RT In(KL)
us
change (ΔS0, Jmol-1K-1), and enthalpy change (ΔH0, kJmol-1). They can be determined
In(KL) = ΔS0/R - ΔH0/RT
(9);
(10).
an
Where KL is the Langmuir equilibrium constant (l/mol); R and T represent the universal gas constant (8.314JK-1mol-1) and the system temperature (K), respectively.
M
ΔG0 can be easily determined from Eq.(9). ΔS0 and ΔH0 are determined from the intercept and slope of the van’t Hoff plots of In(KL) versus 1/T.
negative
at
298.15
K,
d
The ΔG0 (-10.68 KJ/mol) of the adsorption of MB onto the PZS nanotubes is confirming
this
adsorption
is
spontaneous
and
te
thermodynamically favorable. Besides, it has been demonstrated that ΔG0 is less than
Ac ce p
0 drastically suggesting that the driving force of adsorption is greater and the adsorption capacity is higher [35]. The values of ΔS0 and ΔH0 are 66.81 Jmol-1K-1 and
9.84 kJ/mol, respectively. Researchers have reported that the ΔH0 of physisorption is
smaller than 40 KJ/mol [37], implying MB adsorption onto the PZS nanotubes is a physisorption process. And the value of ΔH0 is positive, suggesting the adsorption reaction is endothermic. It is also demonstrated by the temperature effect mentioned above that adsorption capacities increase with the increasing of system temperature. The positive ΔS0 means the degrees of freedom increase at the adsorbent-adsorbate
interface during the adsorption process. All the thermodynamic parameters (ΔG0, ΔH0 and ΔS0) indicate that MB adsorption onto the PZS nanotubes is favorable and PZS nanotubes can be used as an excellent adsorbent to remove MB from aqueous solution.
13
Page 13 of 35
4. Conclusions In this study, the PZS nanotubes were successfully prepared by a facile precipitation polymerization method under mild conditions. The as-synthesized PZS nanotubes which own hollow tubular structures, relatively high specific surface area,
ip t
pore structures, numerous hydroxyl groups, aromatic rings and electron-rich N and O
atoms have been proved to be an excellent absorbent for the removal of MB from could reach up to
cr
aqueous solution. The adsorption capacity at equilibrium at 25
69.16 mg/g and the corresponding contact time was only 15 min which was shorter
us
than the vast majority of adsorbents. Results also showed that MB adsorption onto the PZS nanotubes was highly dependent on temperature, concentration and pH of MB In
the
kinetic
studies,
the
pseudo-first-order
an
solution.
kinetic
model,
pseudo-second-order kinetic model and intraparticle diffusion model were used to fit
M
adsorption data. The pseudo-second-order kinetic model could better describe adsorption kinetics and intraparticle diffusion model also demonstrated the
d
intraparticle diffusion was not the rate-limiting step. The R2 (0.9933) of the Langmuir isotherm and its monolayer adsorption capacity (72.83mg/g) which is highly
te
approaching experimental data (69.16 mg/g), indicated that the adsorption of MB onto
Ac ce p
the PZS nanotubes followed the Langmuir isotherm. The values of thermodynamic parameters (ΔG0, ΔH0 and ΔS0) suggested that MB adsorption onto the PZS nanotubes
was endothermic and spontaneous. Besides, it was a physisorption process. It is expected that this study might develop a novel and efficient adsorbent based on PZS nanotubes for removal of dyes from aqueous solution, and other polymer-based nanomaterials could also been applied.
Acknowledgments We are grateful to the National Natural Science Foundation of China (No. 51003098, 51173170), the Foundation of Henan Educational Committee for Key Program of Science and Technology (No. 12A430014, 14B430036), and the financial support from the Program for New Century Excellent Talents in Universities (NCET).
14
Page 14 of 35
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ip t
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ip t
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Page 18 of 35
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ip t
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19
Page 19 of 35
Tables
Model parameters
R2
Pseudo-first order
qe,cal = 18.56 mg/g k1 = 0.0755 min-1
0.7538
Pseudo-second order
qe,cal = 74.85 mg/g k2 = 0.0067 g mg-1 min-1
0.9991
Intraparticle diffusion
ki1 = 11.41 mg/g min1/2 c1 = 24.82 mg/g ki2 = 0.8634 mg/g min1/2 c2 = 65.54 mg/g
us
cr
Models
ip t
Table 1. Kinetic models for MB adsorption onto the PZS nanotubes.
0.9984
Ac ce p
te
d
M
an
0.9101
20
Page 20 of 35
Parameters
R2
Freundlich
kF = 28.0 L/mg n = 4.555
0.9192
Langmuir
q0 = 72.83 mg/g kL = 0.2444 L/mg
0.9933
Ac ce p
te
d
M
an
us
cr
Isotherms
ip t
Table 2 Isotherm parameters for MB adsorption onto the PZS nanotubes.
21
Page 21 of 35
Table 3 Comparison of saturated adsorption capacities for MB on different adsorbents at the condition of 298 K and pH which is close to 7. q (mg/g)
References
Carbon nanotubes
35.4
[1] [16]
Polyurethane foam
[23]
us
[17]
cr
Magnetite loaded multi-wall 48.06 carbon nanotubes Zeolite 10.86
ip t
Adsorbents
23.03
[25]
4.8
[29]
PZS nanotubes
69.16
This work
Ac ce p
te
d
M
Polyaniline nanotubes
an
Polyaniline nanotubes base/ 5.38 silica composite
22
Page 22 of 35
Captions to Figures Fig. 1. SEM (a) and TEM (b) images of the as-prepared PZS nanotubes.
Fig. 2. (a) FTIR spectrum of the PZS nanotubes. (b) Schematic illustration of the
ip t
preparation of the PZS nanotubes (top). The polycondensation of comonomers HCCP
us
nanotubes (bottom). (P3N3) indicates other phosphazene cores.
cr
and BPS and the highly cross-linked chemical structure of the as-synthesized PZS
Fig. 3. (a) Nitrogen adsorption–desorption isotherm obtained at 77 K for the PZS
an
nanotubes. (b) The pore size distribution curve of the PZS nanotubes.
Fig. 4. (a) Effects of initial pH on the adsorption of MB onto the PZS nanotubes.
M
Conditions: initial MB concentration: 100 mg/L; mass of adsorbent: 15 mg; temperature: 298 K; contact time: 15 min. (b) Zeta potential of the PZS nanotubes at
te
d
different pH values.
Fig. 5. Effects of temperature on the adsorption of MB onto the PZS nanotubes.
Ac ce p
Conditions: initial MB concentration: 100 mg/L; mass of adsorbent: 15 mg; pH: 6.47; contact time: 15 min.
Fig. 6. (a) Effects of initial MB concentration on the adsorption capacity. (b) Effects of initial MB concentration on the adsorption ratio. Conditions: mass of adsorbent: 15 mg; pH: 6.47; temperature: 298 K; contact time: 15 min. (c) The photographs of MB solutions before (left) and after (right) adsorption by the PZS nanotubes.
Fig. 7. (a) Effects of contact time on the adsorption of MB onto the PZS nanotubes. (b) Pseudo-first-order model, (c) Pseudo-second-order model and (d) intraparticle diffusion model for the adsorption of MB onto the PZS nanotubes.
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Fig. 8. (a) Schematic illustration of the adsorption process and mechanism for MB on the PZS nanotubes. (b) FTIR spectra of the PZS nanotubes before and after the MB adsorption, and pure MB.
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Fig. 9. Freundlich (a) and Langmuir (b) isotherms for the adsorption of MB onto the
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Graphical Abstract (for review)
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Graphical abstract
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*Highlights (for review)
Research Highlights 1. Polyphosphazene nanotube as an adsorbent could be facilely synthesized. 2. The adsorbent owns numerous electron-rich N and P atoms and hydroxyl groups.
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3. The adsorbent was an efficient and specific adsorbent for the removal of MB. 4. The pseudo-second-order model could be better to describe the adsorption of MB.
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5. The MB adsorption onto PZS nanotubes was endothermic and spontaneous.
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Figure 9
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