Carbon nanotubes-based polymer nanocomposites: Bio-mimic preparation and methylene blue adsorption

Carbon nanotubes-based polymer nanocomposites: Bio-mimic preparation and methylene blue adsorption

Journal Pre-proof Carbon nanotubes-based polymer nanocomposites: Bio-mimic preparation and methylene blue adsorption Defu Gan, Jibo Dou, Qiang Huang, ...

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Journal Pre-proof Carbon nanotubes-based polymer nanocomposites: Bio-mimic preparation and methylene blue adsorption Defu Gan, Jibo Dou, Qiang Huang, Hongye Huang, Junyu Chen, Meiying Liu, Hongxu Qi, Zhenyu Yang, Xiaoyong Zhang, Yen Wei

PII:

S2213-3437(19)30648-7

DOI:

https://doi.org/10.1016/j.jece.2019.103525

Reference:

JECE 103525

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

27 August 2019

Revised Date:

2 November 2019

Accepted Date:

5 November 2019

Please cite this article as: Gan D, Dou J, Huang Q, Huang H, Chen J, Liu M, Qi H, Yang Z, Zhang X, Wei Y, Carbon nanotubes-based polymer nanocomposites: Bio-mimic preparation and methylene blue adsorption, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103525

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Carbon nanotubes-based polymer nanocomposites: bio-mimic preparation and methylene blue adsorption Defu Gana, Jibo Doua, Qiang Huang a, Hongye Huanga, Junyu Chen a, Meiying Liua, Hongxu Qib, Zhenyu Yang a,* [email protected], Xiaoyong Zhang a,* [email protected], Yen Weic,d,* [email protected]

a

College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

b

Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218,

c

Department of Chemistry and the Tsinghua Center for Frontier Polymer Research, Tsinghua

University, Beijing, 100084, P. R. China. d

Department of Chemistry and Center for Nanotechnology and Institute of Biomedical Technology,

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Chung-Yuan Christian University, Chung-Li 32023, Taiwan

These authors are corresponding authors; Xiaoyong Zhang, Yen Wei, Zhenyu Yang

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Graphical abstract

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USA.

Abstract 1

The effective removal of environmental pollutants using nanomaterials and their composites has attracted great attention over the past few decades. Among them, carbon nanotubes (CNTs) have been extensively applied as adsorbents to remove various environmental pollutants for their small size, large surface areas and unique chemical structure. However, the adsorption performance of pristine CNTs is largely limited for their poor dispersibility and lack of functional groups. In this work, CNTs based composites ([email protected](S-co-MA-DA)) have been successfully synthesized through combination of ring-opening reaction and mussel inspired chemistry. The pristine CNTs and as-prepared [email protected](S-co-MA-DA) were characterized by transmission electron microscope (TEM), Fourier

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transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The influence of various experimental factors on the adsorption process, such as

contact time, initial MB concentration, solution pH and temperature, was studied. Experimental results show that adsorption capacity of [email protected](S-co-MA-DA) is about 4 times that of pristine CNTs in

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same experimental conditions. The kinetics and isotherms studies suggest that pseudo-second-order

kinetic and Freundlich isotherm models could well fit with the adsorption data. The thermodynamic

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studies imply that the adsorption process of MB onto [email protected](S-co-MA-DA) is endothermic and spontaneous. The adsorption mechanism might be the synergistic action of physical adsorption of

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[email protected](S-co-MA-DA) particles and electrostatic interaction between the MB and functional groups on the surface of [email protected](S-co-MA-DA) composites, including carboxyl, acylamino,

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hydroxyl and aromatic moieties. These obtained results and facts indicate that the as-prepared [email protected](S-co-MA-DA) could be utilized as high-performance adsorbents with great potential

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applications in environmental treatment.

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Key words: Carbon nanotubes; Ring-opening reaction; Dopamine; Mussel inspired chemistry; Methylene blue; Adsorption. 1 Introduction

With the rapid development of textile, printing and dyeing industry, a large amount of organic dyes have been discharged in the environment, resulting in major contaminants of water and causing serious environmental problems [1-3]. Therefore, effective removal of these organic dyes from the wastewater has become an important research topic [4-7]. Previously, a number of methods, such as 2

electrochemical methods, chemical precipitation, ion exchange, catalytic reduction, and adsorption etc. have been utilized for removal of these organic dyes [8-16]. Comparing with adsorption behavior towards dyes by nanocomposites, above-mentioned methods are all of different limitations, which include high operating cost, low efficiency and causing second pollution [17-22]. Thus, selection of efficient processing methods plays a significant role in the removal of dyes from wastewater. Adsorption has been considered as a highly-effective method for removal of dyes owing to its efficiency, simple, feasibility and cost-effectiveness. The most important factor for adsorption is to select adsorbents containing large number of active sites. Among many adsorbents, nanocomposites

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have attracted wide attention previously [23-25]. Unfortunately, most of these adsorbents have relatively low adsorption capacity. Therefore, improving adsorption capacity of adsorbents has

acquired great significance in recent years. Surface modification of adsorbents to improve their adsorption capability has been intensively studied recently [26-30].

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Since the first discovery of carbon nanotubes (CNTs) in 1991, CNTs with tubular microstructure have attracted worldwide research interest with various applications ranging from drug delivery,

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electronic devices to catalysis and many others [31-34]. Although CNTs have made great progress in environmental applications, the effect of unmodified CNTs in adsorption application is less than

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satisfactory on account of their agglomeration and lack of active adsorption sites [35, 36]. The pristine CNTs are consisting of hydrophobic carbon atoms and their surface is lack of functional groups.

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Accordingly, the development of facile methods for surface modification of CNTs to improve their dispersity and adsorption capacity is extremely promising [37, 38]. Both the covalent and non-covalent strategies have been reported for the surface modification of CNTs. Covalent modification of CNTs is

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the mainstream method that has been widely accepted and applied in academic, which can improve their dispersion. However, the covalent modification of CNTs severely destroys the original structure of

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CNTs and the involvement of hazardous agents, harsh reaction conditions and expensive equipment, complex procedure, time consuming and low efficient [39]. For example, Gupta et al. [40] have reported that the multiwalled CNTs could be surface functionalized with carboxyl groups and the adsorption capability of these functionalized CNTs could be obviously improved as compared with the unmodified CNTs. Wang et al. [41, 42] have demonstrated that the plasma treatment of CNTs could also greatly improve their adsorption performance toward different environmental pollutants. Non-covalent modification methods are relied on the weak interaction between the CNTs and modified 3

agents, they could well maintain the structure of CNTs, while these modified CNTs are often not stable under some conditions [43-45]. Therefore, seeking simple and effective modification methods is highly desirable to improve their dispersion and adsorption capacity in environmental application. Messersmith and his co-workers found bionic compound of mussel and established mussel inspired chemistry, which provided a novel surface modification strategy for almost all materials and surface regardless of their composition, size and morphology [46-50]. The application of mussel inspired chemistry in surface modification was reported by Lee et al. in 2007. They demonstrated that dopamine (DA) could be self-polymerized under alkaline conditions and formed the polydopamine (PDA)

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coating on the material surface [46-48, 51-53]. More importantly, specific functional groups, including amino and hydroxyl groups, could be introduced on the material surface and improved the

hydrophilicity of materials. These functional groups could also be used for the subsequent conjugation reactions and various polymerizations. Our recent reports have demonstrated that these functional

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composites fabricated from mussel inspired chemistry could be widely utilized for biomedical and

environmental applications [51, 53-56]. However, the surface modification of nanomaterials through

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the functional copolymers pendant with DA molecule is rarely reported [57, 58]. In this study, the DA containing copolymers were facilely synthesized through the ring-opening

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reaction between DA and poly(styrene cooperate maleic anhydride) (poly(S-co-MA)) that were directly applied to modify CNTs to obtain functionalized CNTs composites ([email protected](S-co-MA-DA)) [46].

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Firstly, poly(S-co-MA-DA) copolymers were synthesized through the ring-opening reaction between the anhydride groups of commercial available polymer (poly(S-co-MA) and the amino group of DA (Scheme 1). Secondly, the [email protected](S-co-MA-DA) was prepared through self-polymerization of

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DA under alkaline conditions. Hence, the synthesized functional CNTs based composites show great water-dispersible. Furthermore, the functionalized [email protected](S-co-MA-DA) composites were

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applied as adsorbents to remove methylene blue (MB) under various adsorption parameters that include contact time, temperature, pH and initial MB concentration. The results demonstrated that the adsorption capacity of these modified CNTs towards MB is about 4 folds as compared that of pristine CNTs. The enhancement of adsorption capacity is likely ascribed to the introduction of carboxyl groups on CNTs. More importantly, owing to the universal adhesion of DA to different materials and surface, this method could also be utilized for surface modification of various polymer composites for different applications. 4

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Scheme 1 The synthesis of poly(S-co-MA-DA) through the ring-opening reaction.

2 Experimental sections 2.1 Materials and methods

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Multi-walled carbon nanotubes (CNTs) were purchased from Chengdu Organic Chemicals Co., Ltd. (Chinese Academy of Sciences) and used without any purification. The dopamine hydrochloride

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(denoted as DA) (>98%) was purchased from Sangon Biotech (Shanghai) Co., Ltd. Poly (styrene cooperate maleic anhydride) (poly(S-co-MA)), anhydrous dimethyl sulfoxide (DMSO) and

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tris-(chydroxymethyl)-aminethane (Tris) (>99.9%) were purchased from Aladdin Industrial Co., Ltd (Shanghai, China). Deionized water was prepared for use in solutions. MB was also purchased from

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Aladdin.

The adsorbents ([email protected](S-co-MA-DA)) were synthesized through combination of mussel inspired chemistry and the ring-opening reaction with anhydride. In details, the poly(S-co-MA) (2.0259

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g) and dopamine hydrochloride (2.9354 g) were first dissolved in anhydrous DMSO (35 mL) and heated at 80 °C until they are completely dissolved in DMSO with a magnetic stir bar under the

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nitrogen flow conditions. Then, the reaction continued at room temperature for 30 h with the same conditions. After the reaction system was cooled to ambient temperature, the final reaction mixture was precipitated and purified in methanol and this operation is repeated three times. The purified product was dried at room temperature for further experiments. The CNTs (0.5 g) was dispersed in distilled water for next reaction using ultrasonic treatment. The obtained poly(S-co-MA-DA) (0.5 g) was dissolved and adjusted to be pH 8-9 with the Tris buffer solution using the ultrasonic treatment. The CNTs suspension and homogeneous polymer solution were mixed by magnetic stirring. The solution 5

pH of mixed reactant solution was adjusted to be 8.5 with Tris buffer solution. The resulting suspension was stirred at room temperature for 8 h. After the end of reaction, the obtained product was centrifuged and washed with water and ethanol several times. The final product was dried at 333 K for 24 h. 2.2 Characterization of the samples The morphological structure of CNTs and [email protected] (S-co-MA-DA) were characterized with a JEM-2100 transmission electron microscope (TEM) (Japan electron optics laboratory Co., Ltd.). The FT-IR spectroscopy was applied to the analysis of the surface functional groups by using a Nicolet 5700 Fourier transform infrared spectrometer (FT-IR) instrument (Nicolet Instrument Company, USA)

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over the range from 400-4000 cm-1. The thermo-gravimetric analysis (TGA) was carried out with the thermo-gravimetric analyzer instrument (Q50 V20.10 Build 36) in the nitrogen flow at a heating rate of 20 °C/min with temperatures ranging from 30 to 800 °C. The XPS measurements were utilized to analyze the elemental analysis of the composition and the chemical status of the CNTs and

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[email protected](S-co-MA-DA) using the XPS instrument (ESCALAB250Xi) (Thermo Fisher Scientific Inc. USA) with Al Kα X-ray source.

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2.3 Adsorption experiments

The batch adsorption experiments were carried out to evaluate the application in environment

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treatment by using as-prepared samples to adsorb methylene blue from aqueous solution. After adsorption, the values of methylene blue concentrations were analyzed by using an ultraviolet and

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visible spectrophotometer (TU 1810 pc, Beijing Purkinje General Instrument Co.) at 664 nm (λmax of MB). The amount of adsorbed MB Qt (mg/g) (1) at time t (min) was calculated according to following equation:

C0 Ct m

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Qt

V

(1)

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The equilibrium adsorption capacities Qe (mg/g) (2) and removal efficiencies (R (%)) (3) were

calculated form the following equations:

Qe

R(%)

C0 Ce m

C0 Ce C0

V

(2)

100

(3)

Where C0 and Ce (mg/L) are the initial and equilibrium MB concentration, respectively. Ct (mg/L) is 6

the final (after adsorption) concentrations of MB at time t (min). V (mL) is the volume of MB solution and m (mg) is the mass of adsorbent. 3 Results and discussion 3.1. Characterization of the adsorbents Fig. 1 shows the TEM images of the pristine and functionalized CNTs. From Fig. 1A, the surface of the pristine CNTs is especially smooth and the diameter of themselves is about 30-50 nm, which is consistent with the information from manufacture. As shown in Fig. 1B, the polymers attached on the surface of CNTs can be clearly observed after modification, which demonstrates that CNTs have been

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successfully modified with poly(S-co-MA-DA). More importantly, compared with the pristine CNTs, the structure of the functionalized CNTs is not destroyed and their properties are preserved as indicated from the TEM images. Besides, the method that modifies the pristine CNTs is rather facile and

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effective.

Fig. 1 TEM images of pristine CNTs (A) and functionalized CNTs (B), the scale bar is 100 nm.

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The FT-IR spectroscopy was applied to investigate the surface functional groups of [email protected](S-co-MA-DA). As shown in Fig. 2, no specific adsorption peaks were observed in the

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sample of pristine CNTs [59]. Nevertheless, it was obviously to see that many new adsorption bands appeared after modification from the FT-IR spectrum of [email protected](S-co-MA-DA). The broad

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adsorption band at 3446 cm-1 is attributed to the O-H stretching vibrations of hydroxyl groups and N-H stretching of amino from PDA in the [email protected](S-co-MA-DA). The bands at 2971 and 2907 cm-1 are related to the C-H stretching vibration of PDA backbone. The new adsorption at 1655 cm-1 is assigned to the C=O stretching vibration. The sharp band at 1380 cm-1 is ascribed to the C-N stretching vibration from amide groups. The band at 1035 cm-1 is assigned to the aromatic rings in the PDA. These facts provide further evidences for the successful preparation of [email protected](S-co-MA-DA).

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Fig. 2 Characterization of the as-prepared samples: FT-IR spectra of pristine and functionalized CNTs.

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Fig. 3 shows the TGA curves of pristine and functionalized CNTs in the temperature range from 30 to 800 °C. It can be observed that the weight loss percentage of pristine CNTs was 0.92% under the

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nitrogen flow condition when the temperature was up to 800 °C, which indicated that pristine CNTs possess high purity, good thermal stability and lack of functional groups [60]. After modification, there

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is significant weight loss observed at the temperature range from 30-800 °C. The value of mass loss is calculated to be 32.8%. According to related calculation, the ratio of poly(S-co-MA-DA) to CNTs is nearly about 1:2.1. The weight percentage of decomposed poly(S-co-MA-DA) could be calculated to

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be 32.17%. Such a significant change of weight loss fully indicates that the poly(S-co-MA-DA) is

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successfully attached on the pristine CNTs through the mussel inspired chemistry.

Fig. 3 Characterization of the as-prepared samples. TGA curves of pristine and functionalized CNTs. 8

The significant weight loss between the pristine CNTs and polymer modified CNTs clearly indicates that the DA-containing polymer has been attached on the surface of CNTs through mussel inspired chemistry.

The XPS spectroscopy was also applied to the analysis of the surface state of the pristine and functionalized CNTs (Fig. 4). The predominant elements, including carbon (C), nitrogen (N), oxygen (O), were observed on the sample of [email protected](S-co-MA-DA) from the Fig. 4B-D. As shown in Fig. 4, the binding energies peaks for C 1s, N 1s and O 1s were detected at 284.75, 400.20 and 533.10 eV,

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respectively. As compared with pristine CNTs, the relative intensities of N 1s and O 1s of [email protected](S-co-MA-DA) have remarkable increase, especially for N 1s (from Fig. 4C-D). These

facts suggest that the pristine CNTs were successfully functionalized with the poly(S-co-MA-DA). The major element information of the pristine and functionalized CNTs are displayed in the Table S1. As

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shown in Table S1, the atom percentage of C decreases from 96.1% to 77.5% while that of O and N increase from 3.9% and 0% to 18.16% and 4.34%, which results from the introduction of the

A

C 1s

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B

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poly(S-co-MA-DA). The fact is well consistent with the above-mentioned results.

N 1s

O 1s

D

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Fig. 4 (A) XPS wide scan spectrum and narrow scan spectra of the pristine and functionalized CNTs: (B) C 1s, (C) N 1s and (D) O 1s.

3.2 Adsorption experiment studies 9

3.2.1 The effect of contact time and adsorption kinetics To study the adsorption properties of the as-prepared adsorbents, the batch adsorption experiments, including adsorption experiments on the effect of contact time, concentration, solution pH and temperature, were conducted. As shown in Fig. 5, the effect of contact time on the adsorption capacities is objectively and clearly displayed in Fig. 5A. It is obvious that the adsorption capacities of pristine and functionalized CNTs increases rapidly when the increasing contact time ranges from 0 to 8 min and the adsorption capacity of functionalized CNTs is about 4 times that of pristine CNTs, which is attributed to the functional polymers of poly(S-co-MA-DA) on the surface of CNTs. After the

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functionalization, the functional groups, including amine, carboxyl groups and aromatic moieties, were introduced to the surface of CNTs and were used as active sites to adsorb the MB from aqueous

solution, which led to the significant increase of adsorption capacity. The equilibrium time for MB

adsorption onto [email protected](S-co-MA-DA) was nearly 120 min, slower than that of pristine CNTs

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(about 25 min). This result could be explained with combined action of physical adsorption and

chemical adsorption between MB and adsorbents. Besides, chemical adsorption via electrostatic

equilibrium time about adsorption process. B

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attraction between MB and surface functional groups (carboxyl, acylamino and hydroxyl) led to more

Fig. 5 (A) The adsorption comparison of pristine CNTs and [email protected](S-co-MA-DA) (Dose: 10 mg,

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solution value: 50 mL, initial concentration of MB: 50 mg/L, pH 7, and T: 298 K); (B) Adsorption kinetics for MB onto [email protected](S-co-MA-DA).

To further explore the process and mechanism of adsorption, the kinetic analysis that includes the

pseudo-first-order, pseudo-second-order and intraparticle diffusion model analysis were carried out in this work. The non-linear form of pseudo-first-order adsorption model can be expressed as following equations: 10

Qt

Qe (1 e

k1t

)

(4)

Here Qe (mg/g) represents the adsorption capacity at equilibrium, which was calculated from the Eq and Qt (mg/g) is the amount of adsorbed MB at time t (min). k1 (min-1) is the rate constants of the pseudo-first-order model. From Fig. 5B, the data of MB adsorption were fitted to the three kinetic models. The calculated parameters obtained from the mentioned-above equation are summarized in Table 1. As shown in Table 1, the high correlation coefficient (R2 = 0.98353) indicates that pseudo-first-order kinetics model can be used to describe the kinetics of adsorption of MB onto [email protected](S-co-MA-DA), and the calculated Qe from pseudo-first-order model is found to be much

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closed to the experimental value.

The non-linear form of pseudo-second-order adsorption model rate expression is described as follows:

k2Qe2t 1 k2Qet

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Qt

(5)

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Here k2 (g mg-1 min-1) represents the adsorption rate constant of pseudo-second-order, and the Qt (mg/g) and Qe (mg/g) are the MB adsorption capacity at time t (min) and equilibrium respectively,

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which are based on the assumption that the chemisorption is the rate determining step in the adsorption process. As shown in Fig. 5B and Table 1, the pseudo-second-order shows the correlation coefficient R2 of 0.99813, which is higher than that of pseudo-first-order model (R2 = 0.98353), indicating that the

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adsorption of MB follows the pseudo-second-order rather than the pseudo-first-order. Besides, the adsorption capacity calculated from pseudo-second-order (120.50 mg/g) is closer to the experimental

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results (118.45 mg/g) than that of pseudo-first-order (111.32 mg/g), which is well consistent with above results. As a result, the adsorption of MB onto [email protected](S-co-MA-DA) follows the

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pseudo-second-order model well. Based on these results and kinetic theory, MB adsorption onto [email protected](S-co-MA-DA) composites was chemical and physical adsorption. The intraparticle diffusion kinetic model is usually used to explain the adsorption diffusion

mechanism. The non-linear form of intraparticle diffusion is given as follows:

Qt

k pt 0.5 C

(6)

Here kp (mg g-1 min-0.5) is the rate constant of intraparticle diffusion model and C is the constant associated with the thickness of the boundary layer. Qt (mg/g) is the adsorption capacity at time t (min). 11

As shown in Fig. 5B and Table 1, the correlation coefficient R2 of 0.88265 suggests that the adsorption of MB onto [email protected](S-co-MA-DA) cannot be described with the intraparticle diffusion model, and the curve does not pass through the origin, indicating that the intraparticle diffusion is not the only rate-limiting step in the adsorption process. From Table 1, the value of constant C is calculated at 67.74 mg/g, which implies the great effect of boundary layer on molecule diffusion. Table S2 showed adsorption capacities for MB on [email protected](S-co-MA-DA) and other typical adsorbents. Although there are many excellent adsorbents, adsorption capacity at equilibrium of [email protected](S-co-MA-DA) at 50 mg/L and room temperature could reach up to 118.45 mg/g, which is higher than that of most

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traditional adsorbents.

Table 1 Adsorption kinetic parameters of MB onto [email protected](S-co-MA-DA) at room temperature. Models

Parameters

Initial concentration (mg/L)

Qe (mg/g) K1 (min-1) R2

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Qe (mg/g)

0.98353 120.50 0.001595

h (mg g-1 min-1)

23.17

R2

0.99813

kp (mg g-1 min-0.5)

5.01672

C (mg/g)

67.74

R2

0.88265

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Intraparticle diffusion

0.12146

K2 (g mg-1 min -1)

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Pseudo-second-order equation

111.32

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Pseudo-first-order equation

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50

3.2.2 Influence of initial MB concentration and isotherm analysis Under various initial MB concentrations, the adsorption isotherm models can be constructed through

studying the relationship between equilibrium concentration and equilibrium adsorption capacity. The related adsorption experiments were carried out with the initial MB concentration ranging from 25 to 150 mg/L at room temperature and initial pH 7. As shown in Fig. 6, the adsorption capacity of [email protected](S-co-MA-DA) increases from 74.88 to 155.5 mg/g when the initial MB concentration 12

increases from 25 to 150 mg/L, and the adsorption capacity increases increasingly slowly when the initial MB concentration is greater than 50 mg/L. These results reveal that the more adsorption sites are occupied with the increasing MB molecules, and the adsorption process is gradually saturated. The obtained adsorption experiments data were further analyzed with the Langmuir and Freundlich

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isotherm models.

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Fig. 6 Adsorption isotherms of MB adsorption onto [email protected](S-co-MA-DA) (Dose: 10 mg, solution

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value: 50 mL, initial MB concentration: 25-150 mg/L, pH 7, and T: 298 K).

The Langmuir model assumes that the adsorption takes place in the specific positions of the adsorbents, and no adsorption occurs when the saturation value is reached. The non-linear form of

K LQmCe 1 K LCe

(7)

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Qe

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Langmuir isotherm model is represented as following equation:

Here Qe (mg/g) and Ce (mg/L) are the amount of adsorbed MB per unit weight of adsorbent and MB

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concentration at equilibrium, respectively. Qm (mg/g) calculated from the above equation represents the theoretical saturation adsorption capacity. KL (L/mg) is the Langmuir adsorption constants, which is related to energy of the adsorption. Besides, the dimensionless constant that is an essential feature of the Langmuir isotherm, RL, can be defined as following equation:

RL

1 1 K LC0

(8)

Here C0 (mg/L) is the initial MB concentration. According to the theory of the Langmuir isotherm, 13

the values of RL ranging from 0 to 1 means the adsorption is favorable; RL = 0 suggests the adsorption is reversible; while RL > 1 implies that the adsorption process is unfavorably; RL = 1 indicates that the adsorption is linear. As shown in Table 2, the calculated value of Qm shows that the [email protected](S-co-MA-DA) has a higher adsorption capacity of 189.31 mg/g. The values of RL are found to be within the range of 0.2076-0.6112, suggesting that the adsorption of MB onto [email protected](S-co-MA-DA) is favorable. Freundlich isotherm model, based on an empirical equation used to describe the equilibrium adsorption on the heterogeneous surface, is the one of another widely used model. The model can be described as following equation: 1

K F Cen

(9)

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Qe

Here KF [(mg/g) (L/mg) 1/n] and n-1 are the Freundlich constants related to adsorption energy and

adsorption intensity, respectively. According to the Freundlich model, the smaller value of Freundlich constant of n-1, the better the adsorption performance. If n-1 approaches 0, the adsorption process is

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more heterogeneous. The nature of adsorption is favorable when n-1 is less than 1 yet n-1 exceeding 1

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indicates unfavorable adsorption. According to the calculated results of Table 2, the correlation coefficient of the Freundlich isotherm (R2 = 0.97418) is higher than that of the Langmuir isotherm (R2

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= 0.96729), suggesting that the adsorption behavior of MB onto [email protected](S-co-MA-DA) fits well with the Freundlich isotherm model. The high correlation coefficient from Freundlich model indicates that the adsorption isotherm can also be described with Freundlich model. The value of n-1 is calculated

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to be 0.37337, implying that the adsorption of MB onto [email protected](S-co-MA-DA) is a favorable process. In summary, the high adsorption capacity could be resulted from the rich functional groups of

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polymer poly(S-co-MA-DA).

Table 2 Isotherm parameters for MB adsorption onto [email protected](S-co-MA-DA).

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Isotherms

Langmuir

Freundlich

Parameters

Temperatures (K) 298

Qm (mg/g)

189.31

KL(L/mg)

0.02544

RL

0.2076-0.6112

R2

0.96729

KF[(mg/g)(L/mg)1/n]

23.85264

14

n-1

0.37337

R2

0.97418

3.2.3 The effect of solution pH Fig. 7 shows the effect of solution pH for MB adsorption onto the functionalized CNTs composites. The adsorption capacity increases from 31.43 to 188.98 mg/g with the increasing pH values ranging from 1 to 13, and the removal efficiency increases from 12.57 to 75.59% under the same experimental conditions, implying that the solution pH plays a significant role in the adsorption onto [email protected](S-co-MA-DA). The maximum adsorption capacity of MB by [email protected](S-co-MA-DA)

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was 188.98 mg/g, observed at pH 13. The remarkable increase in adsorption capacity could be explained with the ionization of [email protected](S-co-MA-DA) and MB in solution. At lower pH, H+ ions might compete strongly with the cationic dye MB molecules for the adsorption sites, which may

greatly inhibit the adsorption of MB molecules. Moreover, surface sites, mainly functional groups

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(acylamino, hydroxyl and carboxyl), are protonated, which led to decrease of negatively charged

adsorption sites, resulting from the electrostatic repulsion and the competitive adsorption between H+

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and MB in Fig S3. Under the condition, the adsorption process was mainly governed by chemical adsorption between MB and [email protected](S-co-MA-DA). The increasingly functional groups on the

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surface of adsorbents are converted to negatively charged sites when the solution pH increases, which leads to the remarkable increase in the adsorption of MB due to the electrostatic attraction. This trend

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indicated that adsorption mechanism of MB onto [email protected](S-co-MA-DA) could be mainly involved in electrostatic interactions. These results infer that the solution pH is essential for adsorption

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and provide theoretical guidance to practical application. B

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Fig. 7 (A) The effect of solution pH on the adsorption behavior; (B) removal efficiency for MB adsorption onto [email protected](S-co-MA-DA) (Dose: 10 mg, solution value: 50 mL, initial concentration of MB: 50 mg/L, and T: 298 K). 15

3.2.4 The effect of temperature and adsorption thermodynamics Fig. 8 shows the influence of temperature on the adsorption capacities of MB onto [email protected](S-co-MA-DA). As shown in Fig. 8A, it is obvious that the equilibrium adsorption capacity increases with the increasing temperature, suggesting that the higher temperature is conducive to adsorption of MB onto [email protected](S-co-MA-DA). As the temperature increases from 298 to 348 K, the adsorption capacity of [email protected](S-co-MA-DA) toward MB increases from 113 to 182.5 mg/g. The significant increase of equilibrium adsorption capacity infers that the adsorption of MB onto

B

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A

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[email protected](S-co-MA-DA) is favored at higher temperature.

Fig. 8 (A) The effect of temperature on the adsorption of MB onto [email protected](S-co-MA-DA) (dose:

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10 mg, initial MB concentration: 50 mg/L, pH 7, V: 50 mL); (B) Van’t-Hoff plots of MB adsorption.

To further investigate the adsorption mechanism, these experiment data from the batch adsorption

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experiment were conducted with enthalpy change ΔH0, entropy change ΔS0 and Gibbs free energy change ΔG0. These thermodynamic parameters are determined by the following equations: (10)

H0

T S0

(11)

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G0

Qe Ce

RT ln

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G0

Q S ln e = Ce R

0

H0 RT

(12)

Here Qe (mg/g) and Ce (mg/L) are the equilibrium adsorption capacity and equilibrium MB concentration, respectively. T (K) is the system temperature and R (8.314 J/mol·K) is the universal gas constant. Thermodynamic constants obtained from Fig. 8B were calculated and then listed in Table 3. As 16

shown in Table 3, the values of ΔG0 are found to be negative at different temperatures, implying that the adsorption process of MB onto [email protected](S-co-MA-DA) is spontaneous, and the values of ΔG0 are found to decrease from -1.954 to -3.704 kJ/mol with the increasing temperature, suggesting that the higher temperature is more favorable for MB adsorption. The value of ΔH0 (8.476 kJ·mol-1) implies that the process is endothermic in nature, which confirms that the adsorption reaction does consume energy. Moreover, the heat evolved during physical adsorption falls into the range of 0-40 kJ·mol-1. The magnitude of ΔH0 value obtained in this work is observed in the range of 0-40 kJ·mol-1, suggesting adsorption process is mainly physical adsorption, assisted by chemical adsorption [61]. Based on the

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above results, the adsorption of MB onto [email protected](S-co-MA-DA) is favorable and the adsorbent can be used as a novel adsorbent with great potential application for the removal of MB from aqueous solution.

Table 3 Thermodynamic constants of the adsorption of MB onto [email protected](S-co-MA-DA). ΔG0 (kJ mol-1)

ΔH0 (kJ mol-1)

298

-1.954

8.476

308

-2.304

318

-2.654

328

-3.004

338

-3.354

348

-3.704

ΔS0 (kJ mol-1 K-1)

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T (K)

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0.035

3.2.5 Study on Adsorption Mechanism

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The adsorption mechanism could be researched through FT-IR data and adsorption models. Firstly, FT-IR data could provide some thinking. After adsorption of MB, peaks of [email protected](S-co-MA-DA)

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are weakening, especially adsorption bands at 3446 cm-1 and 1655 cm-1, indicating electrostatic attraction between cationic MB molecules and carboxyl groups. On the other hand, result that solution pH have a key impact on adsorption capacity also proved that electrostatic interactions lead adsorption process. As shown in Fig. S2, aromatic rings adsorption peak at 1035 cm-1 has declined. MB molecules possesses plenty of aromatic rings, which is facile to form π-π stacking with aromatic rings on polymer poly(S-co-MA-DA). Secondly, equilibrium time for MB adsorption on [email protected](S-co-MA-DA) is about 120 min, simultaneously involving physical adsorption and chemical adsorption. Based on all 17

above-mentioned results, adsorption mechanism could be synergistic action of physical adsorption of [email protected](S-co-MA-DA) and electrostatic attraction between MB and functional groups on the surface of [email protected](S-co-MA-DA) composites. 4 Conclusions The [email protected](S-co-MA-DA) composites have been successfully synthesized through the combination of ring-opening reaction and mussel inspired chemistry. The obtained [email protected](S-co-MA-DA) composites were applied as novel adsorbents to remove the MB from the aqueous solution. Characterization results from TEM, FT-IR, TGA and XPS provide sufficient

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evidences for the successful functionalization of CNTs with poly(S-co-MA-DA) through the facile mussel inspired chemistry. The influences of adsorption factors, such as contact time, initial MB

concentration, solution pH and temperature, were conducted. Compared with the pristine CNTs, the

[email protected](S-co-MA-DA) showed much higher MB adsorption capacity for the functional groups on

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the surface of functionalized CNTs composites. Moreover, the adsorption of MB onto

[email protected](S-co-MA-DA) is found to be highly dependent on the solution pH and temperature. The

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pseudo-second-order is the best-fit kinetic model to describe the adsorption process and the Freundlich isotherm could fit well with the equilibrium data from the kinetic results. The thermodynamic study

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suggests that the adsorption of MB onto [email protected](S-co-MA-DA) is an endothermic and spontaneous adsorption process. In conclusion, the adsorption process is governed by the physical

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adsorption and chemical adsorption. These results demonstrated that [email protected](S-co-MA-DA) could be used as an efficient and potential adsorbent in removal of MB and other dye contaminants from wastewater and the strategy of modifying pristine CNTs could be developed as an efficient and

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practicable method to prepare adsorbents with high adsorption capacity.

Conflict of interest

We have no conflict of interest to declare.

Acknowledgments This research was supported by the National Natural Science Foundation of China (Nos. 21865016, 18

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21788102, 21564006, 21561022, 51561022, 21644014).

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