Synthesis of nanocomposites using xylan and graphite oxide for remediation of cationic dyes in aqueous solutions

International Journal of Biological Macromolecules 137 (2019) 886–894

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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac

Synthesis of nanocomposites using xylan and graphite oxide for remediation of cationic dyes in aqueous solutions Chenglong Fu, Xiaobin Dong, Shoujuan Wang ⁎, Fangong Kong ⁎⁎ State Key Laboratory of Biobased Material and Green Papermaking, Key Laboratory of Pulp & Paper Science and Technology of Ministry of Education, Qilu University of Technology, Shandong Academy of Sciences, Jinan 250353, China

a r t i c l e

i n f o

Article history: Received 30 March 2019 Received in revised form 4 July 2019 Accepted 4 July 2019 Available online 05 July 2019 Keywords: Xylan Graphite oxide Adsorbents

a b s t r a c t Due to the rapid development of industrialization, the water resources on which we depend are facing unprecedented challenges. Dyes, as an indispensable substance in our lives, have caused great pollution to the water resources in nature, and the removal of dyes from wastewater is becoming an important topic. A porous xylan/poly (acrylic acid)/graphite oxide nanocomposite was prepared by graft polymerization and used for adsorption of cationic ethyl violet dye in wastewaters in this paper. Various techniques, i.e., Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, elemental analysis, scanning electron microscopy, and ultraviolet-visible spectroscopy, were used to study this composite. Adsorption isotherm measurements showed that the composite's adsorption behavior fits the Langmuir isotherm adsorption model. Adsorption tests showed that this material has excellent adsorption properties; the maximum adsorption capacity for ethyl violet dye was 273.99 mg/g. Investigation of the adsorption mechanism indicated that electrostatic forces and π–π effects are mainly involved in adsorption. Desorption cycling tests showed that the adsorption efficiency of the composite was still over 95% after 3 cycles. These results show that this porous xylan/poly (acrylic acid)/graphite oxide nanocomposite has potential applications in cationic dye removal. © 2019 Published by Elsevier B.V.

1. Introduction Graphite oxide (GO), which was first prepared, by a simple method, in 1958 [1], has shown great potential in fields such as optics, electricity, mechanics, thermodynamics, and physical chemistry [2], and has a wide range of applications, e.g., in sensors, supercapacitors, batteries, and high-efficiency catalysts [3,4]. GO can be combined with other materials such as metals, metal oxides, polymers, and bio-based materials or solutions to form graphite-based composites [5]. Because of rapid industrialization, the water resources on which we depend are facing unprecedented challenges. Various types of industrial wastewater, which contain organic and inorganic chemicals, heavymetal ions, and radioactive substances, are discharged into rivers, causing severe water pollution. Synthetic dyes are a major water pollutant. The textile, papermaking and printing, leather, pharmaceutical, and other industries depend highly on synthetic dyes [6,7]. These industries also use large quantities of water. This has led to the production of large

⁎ Corresponding author. ⁎⁎ Corresponding author at: State Key Laboratory of Biobased Material and Green Papermaking, Key Laboratory of Pulp & Paper Science and Technology of Ministry of Education, Qilu University of Technology, Shandong Academy of Sciences, Jinan 250353, China. E-mail addresses: [email protected] (S. Wang), [email protected] (F. Kong).

https://doi.org/10.1016/j.ijbiomac.2019.07.037 0141-8130/© 2019 Published by Elsevier B.V.

amounts of dye-containing wastewaters. Toxic dyes impede the growth of, and can cause mutations in, aquatic organisms, and therefore pose a serious threat [8]. The removal of dyes from water is therefore an important topic in environmental protection. A number of methods have been developed to solve this problem, e.g., ion exchange, ultrafiltration, flocculation, photocatalysis, adsorption, and electrodialysis [9]. Adsorption is an efficient method for dye removal and is considered to be one of the most promising techniques. GO can be used to adsorb and desorb various atoms and molecules. It has the advantages of a large theoretical surface area and a high negative charge density. However, GO is poorly dispersible in water, easily aggregates, or lacks other required properties, therefore for practical applications it is often prepared as a composite, e.g., GO/metal–organic framework composites [10,11], Ag–TiO2/reduced GO nanocomposites [12], GO/SA/PAM composites [13], CCGO composites [14], and Fe3O4/ Carboxylate Graphene Oxide nanocomposites [15]. Xylan is abundantly available, has good hydrophilicity, and is environmentally friendly, therefore it is used as a flocculant [16], hydrocolloid [17], and wastewater adsorbent [18]. PAA is a hydrophilic polymer. Xylan and PAA could therefore be used to improve the adsorption properties of GO. There are few reports of GO modification with xylan and PAA to produce adsorbents. In this study, we prepared a porous GO nanocomposite modified with xylan and PAA. The strategy for synthesis of the xylan/PAA/ GO nanocomposite is shown in Fig. 1. This composite gave excellent

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Figure 1. Synthesis of xylan/PAA/GO nanocomposite.

adsorption performances in a series of adsorption experiments. The adsorption mechanism was also investigated. In the current study, our goal was to use an environmentally friendly biomass material on GO materials to enhance its adsorption active sites. The strategy for synthesizing xylan/PAA/GO nanocomposite is shown in Fig. 1. The xylan/PAA/GO product was characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Elemental analysis and Scanning electron microscopy (SEM). The xylan/PAA/ GO nanocomposite was used as adsorbent in ethyl violet dye adsorption experiments. The novelties of this work are as follow: 1). prepared a porous xylan/PAA/GO nanocomposite, 2). carried out the adsorption experiments of this nanocomposite on ethyl violet dye solution, and found that it has a high adsorption capacity, 3). the xylan/PAA/GO nanocomposite showed a high reusability and the adsorption efficiency is still higher than 95% after 3 cycles. The

excellent performance of this nanocomposite shows that it has a good potential for wastewater treatment applications. 2. Experimental 2.1. Materials Xylan (from corn cob), ethyl violet, and PAA [average molecular weight (MW) ~450,000] were obtained from the Aladdin Industrial Corporation. Graphite powder, KMnO4, NaNO3, H2O2 (30%), ptoluenesulfonyl chloride, ethylenediamine (Sinopharm Chemical Reagent Co., Ltd.), and acetonitrile are basic laboratory chemicals and are readily available. 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC; Bide Pharmatech Ltd.) and N-hydroxysuccinimide (NHS) were used as a combined catalyst. All materials were analytical grade and used without further purification. Deionized water, which

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2.2.2. Preparation of xylan/PAA PAA (0.5 g) was added to deionized water (50 mL), and the mixture was stirred until the solution was clear. The solution was cooled to below 10 °C and then EDC (1 g) and NHS (0.3 g) were added. The mixture was stirred thoroughly and xylan-NH2 (3 g) was added. The mixture was stirred for 48 h and then transferred to a dialysis bag. The water in the beaker was continuously changed until its conductivity reached a constant value. The mixture was dried in an oven at 60 °C [21]. 2.2.3. Preparation of GO Hummers method, which is the simplest method for GO preparation, was used [1]. Briefly, concentrated H2SO4 (23 mL) was added to a 250 mL beaker containing graphite (1 g) and the mixture was stored at 0 °C in an ice bath. NaNO3 (0.5 g) was dissolved in the mixture, and the mixture was stirred for 3 to 5 min before adding KMnO4 (3 g). The mixture was stirred for 2 h with a mechanical stirrer at less than 10 °C. The temperature was raised to 35 °C for 12 h and then deionized water (150 mL) was added. The mixture was then heated to 90 °C and stirred for 30 min. H2O2 (50 mL) was added and the mixture was cooled to room temperature. The mixture was washed three times with dilute hydrochloric acid solution and then centrifuged. The product was washed with deionized water until it was neutral and then dried in a vacuum oven. 2.2.4. Preparation of xylan/PAA/GO GO (0.1 g) was added to phosphate buffer solution (PBS, pH 6, 5 mL). The mixture was sonicated for 20 min to achieve uniform dispersion of GO. Xylan-NH2/PAA (0.1 g) was dissolved in the GO suspension under stirring, and the mixture was refrigerated for 30 min at low temperature (about 5 °C). The mixture was mixed with EDC (0.03 g) and NHS (0.01 g) under stirring at low temperature (about 5 °C) for 1 h, and then allowed to react at room temperature for 48 h. The product was separated by centrifugation under refrigeration at a rotational speed of 5000 r/min, washed at least three times with deionized water, and dried at 60 °C [22]. 2.3. Batch adsorption experiments

Figure 2. FTIR spectra of xylan, xylan-Ts, xylan-NH2, PAA, GO, xylan/PAA, and xylan/PAA/ GO.

was supplied by a water purification system, was used in all experiments.

2.2. Preparation of xylan/PAA/GO 2.2.1. Preparation of xylan-NH2 Xylan (5 g) was dissolved in water (80 mL), and then NaOH solution was slowly added with a pipette. An acetonitrile solution (5 mL) of ptoluenesulfonyl chloride (1.3 g) was mixed with the xylan aqueous solution and stirred at room temperature for 3 h. The mixture was filtered and the pH of the filtrate was adjusted to neutral. The filtrate was kept overnight (about 10 h) under cryopreservation (about 4 °C) conditions. The obtained white precipitate (xylan-Ts) was thoroughly washed with acetone and dried in an oven at 60 °C. The obtained xylan-Ts (1 g) was dissolved in anhydrous ethylenediamine (5 mL, MW 60.10). The mixed solution was heated to 80 °C and the temperature was maintained for 72 h. The solution was cooled to room temperature and then acetone was added to induce precipitation. The precipitate was dissolved in a pre-prepared methanol solution, filtered, and dried in an oven at 60 °C to give xylan-NH2 [19,20].

A series of adsorption experiments were performed under different conditions, i.e., at various pH values, initial dye concentrations, and adsorption times. The adsorbent (5 mg) was added to a 50 mL conical flask and ethyl violet solution (10 mL) of a specific concentration was added with a pipette. The pH was adjusted with 0.1 mol/L acidic and alkaline solutions, and the mixture was shaken for 120 min at room temperature. A certain amount of the suspension was centrifuged (8000 r/min, 10 min) to remove particles. The dye concentration in the supernatant was determined by ultraviolet (UV) spectroscopy. The amount of dye adsorbed (eq. 1) and the dye removal rate (Eq. (2)) were calculated. qe ¼

ðC0 −Ce Þ  V m

removal ð%Þ ¼

C0 −Ce  100% C0

ð1Þ

ð2Þ

where qe (mg/g) is the amount adsorbed per unit of adsorbent, C0 (mg/L) is the initial dye concentration, Ce (mg/L) is the dye concentration after adsorption, V (L) is the dye solution volume, and m (g) is the adsorbent mass [23]. 2.4. Desorption experiments Pure ethanol (95%) was used as a desorption agent to explore the recyclability of xylan/PAA/GO. A certain amount of adsorbent was added to an ethyl violet dye solution (10 mL) of specific concentration, and

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Fig. 3. The XRD patterns of graphite, GO, and xylan/PAA/GO (a), Mass loss and mass loss rates for xylan/PAA (b), GO (c), and xylan/PAA/GO (d).

the mixture was shaken with a shaker for 120 min. After adsorption, the mixture was centrifuged. The dye concentration in the supernatant was determined by UV spectroscopy. The used adsorbent was added to ethanol. The mixture was stirred thoroughly and then filtered. The recovered adsorbent was used for the next adsorption cycle. Adsorption and desorption were performed three times [24,25]. 2.5. Characterization The functional groups in the samples were identified by Fouriertransform infrared (FTIR) spectroscopy. The sample was pressed into a KBr sheet and the FTIR spectrum was recorded (ALPHA) in the range 400 to 4000 cm−1. X-ray diffraction (XRD; XD8 ADVANCE) was performed in the scanning range 5° to 80°. Thermogravimetric analysis (TGA; TGA Q50) was used to investigate the thermal stability of the sample. The sample (5–10 mg) was heated to 800 °C from ambient temperature at a heating rate of 10 °C/min. The C, H, O, and N contents were determined with an automatic elemental analyzer. Scanning electron microscopy (SEM; Hitachi Regulus 8220) was used to examine the sample morphologies, and local elements were determined by energydispersive X-ray spectroscopy (EDS; Xflash 6160). The intensity of the ethyl violet absorption peak was measured at 596 nm by UV–vis spectroscopy (Agilent 8453) to evaluate the adsorption performance of the sample.

Table 1 Elemental analysis results for xylan, xylan-NH2, xylan/PAA, GO, and xylan/PAA/GO. Samples

C, wt%

H, wt%

O, wt%

N, wt%

Xylan Xylan-NH2 Xylan/PAA GO Xylan/PAA/GO

39.35 35.75 40.53 44.08 46.08

8.67 8.43 7.90 8.04 6.66

50.58 42.43 43.40 45.53 42.77

0.05 9.23 4.16 0.04 3.07

3. Results and discussion 3.1. FTIR spectroscopy The functional groups in the synthesized samples were identified by FTIR spectroscopy; the spectra are shown in Fig. 2. In the xylan spectrum, the broad peaks at 3400 and 2904 cm−1 correspond to the \\OH and C\\H stretching vibrations, respectively. The peak at 1638 cm−1 mainly arises from adsorption of water. The peak at 1042 cm−1 is assigned to the C\\O stretching vibration and the peak at 1157 cm−1 indicates the presence of arabinosyl units [26–28]. A new peak appeared in the xylan-Ts spectrum, at 1410 cm−1, and is attributed to the benzene C_C stretching vibration. In the xylan-NH2 spectrum, a peak appeared at 1577 cm−1; this confirms grafting of amino groups onto xylan. In the GO spectrum, the broad peak at 3400 cm−1 is attributed to the \\OH stretching vibration. The peaks centered at 1723 and 1623 cm−1 are ascribed to carboxylic C_O and aromatic C_C stretching vibrations, respectively. The weak peak at 1404 cm−1 is associated with the inplane bending vibration of C\\OH, and the peak at 1048 cm−1 can be assigned to the epoxy group stretching vibration [29,30]. In the xylan/ PAA spectrum, the peaks at 1643 and 1560 cm−1 correspond to the stretching vibration of amide C_O and the C\\N\\H bending vibration, respectively [14]. These peaks confirm that xylan/PAA fabrication was successful. The spectrum of the xylan/PAA/GO nanocomposite contained the characteristic IR absorption peaks of GO and xylan/PAA; this proves that xylan/PAA was successfully grafted onto GO. 3.2. XRD XRD was used to identify the crystalline phases of graphite, GO, and the xylan/PAA/GO nanocomposite; the XRD patterns are shown in Fig. 3a. The XRD pattern for the graphite sample showed a sharp peak at 26.7°; this indicates that the graphite had good crystallinity. The

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Fig. 4. SEM images of GO (a) and xylan/PAA/GO (b), and EDS results for xylan/PAA/GO (c).

graphite interlayer distance, which was calculated with the Bragg equation, was about 0.34 nm. After oxidation of the graphite to GO, the characteristic peak for graphite disappeared and a new peak appeared at 11.1°. This is the characteristic peak of GO. The calculated GO interlayer distance was 0.8 nm. The sharp peak shows that GO retained a high degree of order, and the peak shift from a high angle to a low angle indicates that the distance between the layers increased. These changes confirm that GO was prepared. The peak at 11° was also present in the xylan/PAA/GO pattern. This shows that the crystalline structure of GO was not changed by graft polymerization with xylan/PAA [31,32]. 3.3. TGA Thermal analysis is frequently used to investigate the oxidation and carbonization of materials, and the stabilization and activation of their surface functional groups. TGA gives the change in the mass of a sample with increasing temperature, and differential thermogravimetry (DTG) provides clearly identifiable signals that correspond to each mass change. Fig. 3b, c, and d show the mass changes for xylan/PAA, GO, and xylan/PAA/GO, respectively, with increasing temperature. TGA was performed from room temperature to 800 °C under N2 and the heating rate was 10 °C/min. In Fig. 3b, c, and d, the front of each curve shows a small mass loss (Fig. 3b, mass loss 12.16%; Fig. 3c, mass loss 16.93%; and Fig. 3d, mass loss 9.79%); these losses are related to vaporization of physically adsorbed water molecules on the sample surfaces [33]. The structure of xylan is complex because of the large number of side-chain radicals, and its thermal stability is consequently lower than those of other

biomass materials. The initial decomposition temperature for xylan is therefore about 200 °C. In Fig. 3b, the second peak (310.14 °C) in the DTG curve is ascribed to decomposition of residual xylan side chains [34–36]. In Fig. 3c, the DTG curve shows three peaks (97.72, 226.88, and 284.62 °C), which correspond to three major mass loss stages. These can be related to various functional groups on the GO surface. In Fig. 3d, the four DTG peaks indicate the mass loss stages for xylan/ PAA/GO. The first three peaks (89.36, 221.77, and 281.35 °C) are consistent with those for the mass loss phases for GO, and the last (403.37 °C) is similar to that for the mass loss phase for xylan/PAA in Fig. 3b (408.76 °C). Previous studies have shown that the thermal decomposition of polymer molecules occurs over a wide temperature range [33]. In the carbonization process indicated by TGA, the mass losses for GO (21.6%) and xylan/PAA/GO (27.99%) are similar. This may be because the mass loss is mainly the result of GO carbonization.

3.4. Elemental analysis The C, H, O, and N contents of xylan, xylan-NH2, xylan/PAA, GO, and xylan/PAA/GO were determined using an automatic elemental analyzer. The results (Table 1) show that pure xylan contained almost no N. After ammoniation with anhydrous ethylenediamine, the N content of xylan was 9.23 wt%; this is consistent with the FTIR spectrum. PAA was grafted with xylan-NH2, and the product, i.e., xylan/PAA, contained 4.16 wt% N. The N content of the functionalized GO was 3.07 wt%, which shows that the grafted product was obtained from GO by esterification.

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Fig. 5. (a) Effect of initial pH on removal rate of ethyl violet with xylan/PAA/GO adsorbent; [ethyl violet] = 30 mg/L, m (xylan/PAA/GO) = 5 mg, temperature 25 °C. (b) Effect of initial ethyl violet dye concentration on removal efficiency; m (xylan/PAA/GO) = 5 mg, temperature 25 °C, pH 7.2. (c) Effect of contact time on removal efficiency; [ethyl violet] = 30 mg/L, m (xylan/ PAA/GO) = 5 mg, temperature 25 °C, pH 7.2. (d) Adsorption isotherm of ethyl violet on xylan/PAA/GO; [ethyl violet] = 5–200 mg/L, m (xylan/PAA/GO) = 5 mg, temperature 25 °C, pH 7.2.

3.5. SEM SEM was used to provide clear information on the textures of the samples. EDS analysis provided reliable qualitative information on the elements in the samples. The SEM images in Figure show the morphologies of GO and the xylan/PAA/GO nanocomposite. Fig. 4a shows the basic characteristics of GO, the surface is wrinkled, layered [37], and smooth. Fig. 4b shows dense pores in the xylan/PAA/GO nanocomposite. This pore structure favors adsorption. The EDS (Fig. 4c) results indicate that the xylan/PAA/GO surface contained N, which is consistent with the elemental analysis results.

from 2 to 7. This is because large amounts of oxygen-containing groups such as carboxyl groups and hydroxyl groups are present in the xylan/ PAA/GO adsorbent. These negatively charged oxygen-containing groups in the solution are protonated under acidic conditions. The cations in ethyl violet dye (formed because of the presence of amino groups) compete with H+ for the binding sites in the adsorbent. The stronger the acidity is, the larger the amount of H+. This results in a low dye removal rate. In contrast, the removal rate increases, and can be maintained at a high level, when the pH is greater than 7. This is because the ethyl violet dye can easily combine with the adsorbent when the H+ concentration is low. The lowest pH that enables a high removal rate is 7.2. This nearneutral pH was selected for the adsorption reactions to avoid the effects of an alkaline environment.

3.6. Batch adsorption 3.6.1. Effect of initial solution pH on adsorption In general, the pH of a dye solution is an important factor in adsorption [38]. The pH not only affects the structural stability and colour development of the dye, but also affects the oxygen-containing functional groups and charges on the surface of the adsorbent [39,40]. The removal rates of ethyl violet dye from solutions with different initial pH values but the same amounts of xylan/PAA/GO adsorbent were compared. The results are shown in Fig. 5a. The rate of ethyl violet dye removal from the solution increased rapidly with increasing pH

3.6.2. Effect of initial dye concentration on adsorption Fig. 5b shows the curve of the dye removal rate against initial dye concentration. As the dye concentration increased, the dye removal rate remained at above 95% when xylan/PAA/GO was used as the adsorbent. When the dye concentration was increased to over 120 mg/L, the removal rate decreased significantly. Because the number of active adsorption sites on the adsorbent surface was limited, the removal rate of ethyl violet dye decreased with increasing initial concentration. In addition, a high initial concentration of ethyl violet dye could also have a negative impact on adsorption.

Table 2 Kinetic parameters for adsorption of ethyl violet with xylan/PAA/GO nanocomposite.

Table 3 Parameters in Langmuir and Freundlich isotherm models of ethyl violet at 25 °C.

Rseudo-first-order

Rseudo-second-order

Langmuir

Freundlich

qe (mg/g)

K1 (min−1, ×10−4)

R2

qe (mg/g)

K2 (g/(mg·min))

R2

qmax (mg/g)

b (L/mg)

R2

K (mg/g)

1/n

R2

41.42

6.3517

0.9796

32.16

0.0287

0.9916

273.99

0.1999

0.9927

68.5743

0.3626

0.9117

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Table 4 Comparison of adsorption capacities of various adsorbents. Adsorbents

Adsorption capacity(mg/g)

References

NMWC RSBE xylan/PAA/GO

100.4 40.7 273.99

[40] [42] This work

3.6.3. Adsorption kinetics It is important to determine the adsorption kinetics of an adsorbent to understand the dye adsorption rate and adsorption processes in aqueous solution. The adsorption kinetics can clarify the adsorption reaction pathways and enable exploration of possible adsorption mechanisms. In this study, we used two different models of adsorption kinetics: the Lagergren pseudo-first-order model (Eq. (3)) and the pseudo-second-order model (Eq. (4)) [41]. ln ðqe −qÞ ¼ lnqe −k1 t q2e kt q¼ 1 þ qe k 2 t

ð3Þ ð4Þ

Eqs. (3) and (4) are independent; qe and q (mg/g) are the amounts of dye removed at adsorption equilibrium and at adsorption time t (min); k1 (min−1) and k2 (g/(mg·min) are the rate constants of the Lagergren pseudo-first-order model and the pseudo-second-order model, respectively. Fig. 5c shows the effect of contact time on removal efficiency. It can be seen that the maximum adsorption is about 80 min. The fitting results (Table 2) indicate that the pseudo-second-order model is more in line with our experimental results, and the amount adsorbed at equilibrium is similar to the experimental result. The Lagergren pseudofirst-order model gives an unsatisfactory fit. This is consistent with the

results of other adsorption studies. Although the Lagergren pseudofirst-order model is applicable in the initial stage of the adsorption process, it is inaccurate for the overall process. Any model has errors, and our goal was to reduce the errors and make the fitting more practical. For example, in the model described by Canzano, a high t value gave a high error. Appropriate removal of the adsorption equilibrium point and near-equilibrium data improved the fitting [40]. 3.6.4. Adsorption isotherms The adsorption isotherm equation shows the relationship between the amount of dye adsorbed by the adsorbent and the dye concentration in the solution when adsorption reaches equilibrium at a certain temperature. The two commonly used adsorption isotherm equations are the Langmuir isotherm (Eq. (5)) and the Freundlich isotherm (Eq. (6)) [42,43]. Ce =qe ¼ Ce =ðqmax Þ þ 1=ðbqmax Þ

ð5Þ

qe ¼ KCe 1=n

ð6Þ

where qe (mg/g) is the amount adsorbed at equilibrium, qmax (mg/g) is the maximum adsorption amount, b (L/mg) is the adsorption equilibrium constant, Ce (mg/L) is the concentration of ethyl violet dye, and K and 1/n are empirical constants. The experimental data were fitted to the Langmuir and Freundlich isotherm models. The Langmuir equation is applicable to homogeneous adsorption system based on no interaction between the sorbate molecules. Also there are homogeneously distributed active adsorption sites on the layered nanocomposite flakes, and the dye molecules form a monolayer covering on the surface of the layered nanocomposites, as well as one adsorption site on the solid surface can only adsorb one molecule or atom like a theater seat [42,43]. The data in Table 3 show that the values obtained with the Langmuir isotherm model

Fig. 6. Proposed mechanism of ethyl violet dye removal by xylan/PAA/GO.

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Fig. 7. (a) Comparison of 30, 60, and 100 mg/L ethyl violet dye aqueous solutions and adsorbent after three adsorption cycles and (b) recycling tests for xylan/PAA/GO in three ethyl violet dye solutions of different concentrations; m (xylan/PAA/GO) = 5 mg, temperature 25 °C.

were more in line with the experimental data. Fig. 5d shows the adsorption capacity of the xylan/PAA/GO composite. The maximum adsorption capacity of the xylan/PAA/GO composite was 273.99 mg/g, which is much higher than those of the other adsorbent materials listed in Table 4. The main reason for the high adsorption capacity of the xylan/ PAA/GO composite is the abundant functional groups on the GO surface, which provide a large number of adsorption sites. Furthermore, addition of the xylan/PAA polymer exposes a large number of carboxyl functional groups in the composite and changes the hydrophilicity and dispersibility of GO in water. For dyes with high solubility in the aqueous phase, the increase in dispersibility facilitates binding of the adsorbent to the dyes, and this improves the dye removal efficiency. 3.6.5. Adsorption capacity comparison and removal mechanism Adsorption is the interaction between an adsorbent and an adsorbate. Different types of adsorbent and adsorbate have different properties, and different adsorbent–adsorbate combinations result in different adsorption interactions. These adsorption interactions can be roughly divided into categories such as London dispersion forces, electric dipole–dipole interactions, electric quadrupole interactions, electrostatic forces, hydrogen-bonding interactions, π–π effects, and surface modification. Adsorption is generally considered to be a surface phenomenon. Depending on the type of reaction, adsorption can be divided into physical adsorption and chemical adsorption [44]. In this study, there was no chemical connection between the xylan/ PAA/GO composite and the ethyl violet dye, therefore we concluded that the adsorption process was physical adsorption. Electrostatic forces and π–π effects can be used to explain this adsorption behavior. The abundant functional groups on the GO surface are beneficial for modification and favor adsorption behavior. Because of the hydroxyl groups and carboxyl groups on the GO surface, the carboxyl groups in PAA, and the hydroxyl groups in xylan, the composite had a strong negative charge. Ethyl violet is a cationic dye, therefore electrostatic attraction between the composite and the dye occurred easily in aqueous solution, and this resulted in adsorption. In addition, the aromatic ring structures of the ethyl violet dye and xylan/PAA/GO composite generate a π–π effect, which also leads to adsorption [42]. The proposed removal mechanism is shown in Fig. 6. 3.6.6. Desorption The recycling performance is a key criterion in evaluating adsorbents. Desorption experiments were performed with three ethyl violet dye solutions, of concentrations 30, 60, and 100 mg/L. The adsorbent was recycled three times and the ethyl violet dye removal rates were determined. Fig. 7a shows that the adsorbent still gave a good adsorption performance in solutions of different concentrations after three adsorption cycles. The ethyl violet dye removal rates were calculated. The results show that the dye removal rate by xylan/PAA/GO remained

above 95% after three adsorption cycles (Fig. 7b). The results show that xylan/PAA/GO is a useful adsorbent.

4. Conclusions In this study, a xylan/PAA/GO nanocomposite was synthesized by solution of polymerization of xylan and PAA-grafted GO. The novel composite showed high efficiency as an adsorbent. Xylan, PAA, and GO each has a certain adsorption capacity for cationic ethyl violet dye. The xylan/ PAA/GO nanocomposite adsorbed the cationic ethyl violet dye by electrostatic forces and π–π effects. The composite showed extremely high recycling efficiency. However, there are some problems associated with adsorbent recovery, and this will be the focus of our future work.

Acknowledgement The present work was financially supported by the National Key Research and Development Program of China (No. 2017YFB0308000), the National Natural Science Foundation of China (Grant Nos. 31570566, 31800499). We thank Helen McPherson, PhD, from Liwen Bianji, Edanz Editing China, for editing the English text of a draft of this manuscript. References [1] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [2] K.S. Novoselov, V.I. Fal'ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192–200. [3] R. Rudra, V. Kumar, N. Pramanik, P.P. Kundu, Graphite oxide incorporated crosslinked polyvinyl alcohol and sulfonated styrene nanocomposite membrane as separating barrier in single chambered microbial fuel cell, J. Power Sources 341 (2017) 285–293. [4] R. Jamatia, A. Gupta, B. Dam, M. Saha, A.K. Pal, Graphite oxide: a metal free highly efficient carbocatalyst for the synthesis of 1, 5-benzodiazepines under room temperature and solvent free heating conditions, Green Chem. 19 (2017) 1576–1585. [5] G. Zhao, X. Li, M. Huang, Z. Zhen, Y. Zhong, Q. Chen, X. Zhao, Y. He, R. Hu, T. Yang, R. Zhang, C. Li, J. Kong, J.-B. Xu, R.S. Ruoff, H. Zhu, The physics and chemistry of graphene-on-surfaces, Chem. Soc. Rev. 46 (2017) 4417–4449. [6] H.J. Kumari, P. Krishnamoorthy, T. Arumugam, S. Radhakrishnan, D. Vasudevan, An efficient removal of crystal violet dye from waste water by adsorption onto TLAC/ chitosan composite: a novel low cost adsorbent, Int. J. Biol. Macromol. 96 (2017) 324–333. [7] W. Khanday, M. Asif, B. Hameed, Cross-linked beads of activated oil palm ash zeolite/chitosan composite as a bio-adsorbent for the removal of methylene blue and acid blue 29 dyes, Int. J. Biol. Macromol. 95 (2017) 895–902. [8] D. Rawat, V. Mishra, R.S. Sharma, Detoxification of azo dyes in the context of environmental processes, Chemosphere 155 (2016) 591–605. [9] W. Peng, H. Li, Y. Liu, S. Song, A review on heavy metal ions adsorption from water by graphene oxide and its composites, J. Mol. Liq. 230 (2017) 496–504. [10] S. Wu, L. Yu, F. Xiao, X. You, C. Yang, J. Cheng, Synthesis of aluminum-based MOF/ graphite oxide composite and enhanced removal of methyl orange, J. Alloys Compd. 724 (2017) 625–632. [11] M. Chen, Y. Ding, Y. Liu, N. Wang, B. Yang, L. Ma, Adsorptive desulfurization of thiophene from the model fuels onto graphite oxide/metal-organic framework composites, Pet. Sci. Technol. 36 (2017) 141–147.

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