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Fabrication of the novel hydrogel based on waste corn stalk for removal of methylene blue dye from aqueous solution
Accepted Manuscript Title: Fabrication of the novel hydrogel based on waste corn stalk for removal of methylene blue dye from aqueous solution Authors: Dongzhuo Ma, Baodong Zhu, Bo Cao, Jian Wang, Jianwei Zhang PII: DOI: Reference:
S0169-4332(17)31728-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.072 APSUSC 36272
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-3-2017 23-5-2017 6-6-2017
Please cite this article as: Dongzhuo Ma, Baodong Zhu, Bo Cao, Jian Wang, Jianwei Zhang, Fabrication of the novel hydrogel based on waste corn stalk for removal of methylene blue dye from aqueous solution, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.072 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.
Fabrication of the novel hydrogel based on waste corn stalk for removal of methylene blue dye from aqueous solution Dongzhuo Ma1, Baodong Zhu1,∗, Bo Cao2, Jian Wang1, Jianwei Zhang1 1
Provincial Key Laboratory Oil and Gas Chemical Technology, College of Chemistry and
Chemical Engineering, Northeast Petroleum University, Daqing, Heilongjiang 163318, China 2
Daqing Oil Field Co., Daqing, Heilongjiang 163453, China
∗ Corresponding author. Tel.: +86 13836884622. E-mail address: [email protected] (B.D. Zhu).
Graphical abstract:
Highlights:
The novel hydrogel based on waste corn stalk was successfully fabricated by aqueous solution polymerization technique.
The fine microstructures of the material were clearly observed by Biomicroscope, which was different from the FESEM images.
99.70% MB dye was rapidly removed with the novel hydrogel composite.
The utilization of waste corn stalks was enriched.
It will be engaged in the remediation of industrial effluents.
Abstract The novel hydrogel based on waste corn stalk was synthetized by aqueous solution polymerization technique with functional monomers in the presence of organic montmorillonite (OMMT) under ultrasonic. In this study, batch adsorption experiments were carried out to research 1
the effect of initial dye concentration, the dosage of hydrogel, stirring speed, contact time and temperature on the adsorption of methylene blue (MB) dye. The adsorption process was best described by the pseudo-second-order kinetic model, which confirmed that it should be a chemical process. Furthermore, we ascertained the rate controlling step by establishing the intraparticle diffusion model and the liquid film diffusion model. The adsorption and synthesis mechanisms were vividly depicted in our work as well. Structural and morphological characterizations by virtue of FTIR, FESEM, and Biomicroscope supported the relationship between the adsorption performance and material’s microstructure. This research is a valuable contribution for the environmental protection, which not only converts waste corn stalks into functional materials, but improves the removal of organic dye from sewage water.
Key words: Corn stalk; Clay; Hydrogel; Adsorption; Kinetics
1. Introduction Corn stalk whose main chemical components contain abundant cellulose, hemicellulose and lignin is one of the by-products of grain crops [1]. So far the utilization of waste corn stalk is not high, except for a limited comprehensive utilization as feed, fertilizer, industrial raw materials, fuel, and so forth [2]. Others are mostly incinerated, which not only gives rise to the huge waste of biomass resources, but the serious environment pollutions as well. Cellulose which belongs to a linear and extended polysaccharide stands out as one of the most significant natural polymers [3]. If the corn stalks which reserve 40-60% natural cellulose are applied to the fabrication of cellulose-based hydrogels, the farmland waste will play a crucial role on the field of environmental protection. Those polysaccharide-based biodegradable materials possess attractive advantages like low cost, easily available, nontoxicity, renewability, biocompatibility, environment -ally friendly natures, and so on [4]. However, their applications have been limited on account of the poor mechanical property and low adsorption capacity. To address the problem, many researchers have kept trying to modify these polymers by pretreatment and functionalization [5]. Grafting copolymerization becomes a convenient and efficient approach for chemical modification of cellulose that for instance introducing some special monomers with functional groups such as hydroxyl group, carboxyl group, amino group, sulfo group, onto the backbone of cellulose [6,7]. Copolymerizing different types of monomers and polysaccharides will be a versatile option for joining the excellent properties of both [8], which can eliminate the weakness from the performance of the polysaccharide-based hydrogel [9]. The hydrogels own three-dimensional hydrophilic networks of both natural and synthetic polymers via chemically or physically crosslinking [10]. They are the classic representative of soft materials [11], which has gained extensive attention as biological, agriculture and eco-friendly function materials [2,12-14]. In addition, they are insoluble and possess the ability to absorb and retain a larger volume of water 2
by means of networks and pore structures without losing their physical dimension structure [15,16]. Nowadays, it has been reported that a great deal of industries for instance cosmetic, textile, printing, plastics, paper production, leather, food industries often use dyes in their production processes [17]. Nevertheless, these dye effluents are discharged into the environment without any water treatment. Most of dyes are toxic, carcinogenic and mutagenic, even at low concentrations, which are harmful to the human health [18]. Furthermore, these wastes lower the water quality, which represent risk to aquatic life. The bulk of photosynthesizing species are also under threat due to the restriction in light penetration in dye-contaminated water. Methylene blue (MB), a cationic dye, officially named as 3,7-bis(dimethylamino)-phenothiazin-5-ium chloride, is considered more toxic than those anionic dyes [19]. It can easily permeate into the cells by interacting with negatively charged surface of cell membrane. So, rapid and highly effective removal of MB dye from industrial effluents has been a challenging research project [20]. Up to now, various methods for removal of MB dye from aqueous solution including membrane separation, oxidation or ozone treatment, electrolysis, ion-exchange, chemical precipitation, photocatalytic degradation, adsorption, and the like [21]. Among these technologies, the adsorption has been found to be a kind of flexible and economical approach on the removal of different synthetic dyes [22]. Clays become one of popular materials as adsorbents for dyes currently, like montmorillonite (MMT) consists of layers of two tetrahedral silica sheets sandwiching one octahedral alumina sheet [23]. MMT has been widely used as an adsorbent due to its large specific surface and robust cation-exchange ability [24]. In order to improve the compatibility between MMT and polymer matrix, the chemical modification of hydrophilic silicates is urgently required, so the organo-clays are born at the right moment [25]. In previous studies, natural polymer/clay hydrogel composites had been paid great attention to use them as absorbents for removal of contaminants [26]. Yi et al. [27] successfully prepared a novel hybrid hydrogel using sodium humate, polyacrylamide, and hydrophilic laponite. The adsorption and desorption of MB experiments show that it had the good adsorption capacity and the maximum absorption concentration of MB was 800 mg/L. Shirsath et al. [28] produced the hydrogel composite by incorporation of kaoline clay to remove Brilliant Green dye. It had been observed that the maximum dye removal by PAA-K hydrogel was achieved for the initial dye concentration of 30 mg/L at a temperature of 35 ℃ and pH=7, further, it confirmed the contribution of clay minerals. In addition, the bentonite filler was also introduced into the hydrogel, this nano-clay filled composite hydrogel can efficiently finish the removal of some cationic dyes or metal ions from sewage [29]. Many researchers are devoting themselves to develop the new functional materials for wastewater treatment [30]. Surprisingly, there is less report about wastewater treatment using waste corn stalks. In our study, we selected the corn stalk as an ingredient which grafted diffident functional monomers 3
such as acrylic acid (AA) and acrylamide (AM). Organic montmorillonite (OMMT) was also chosen for the fabrication of the novel hydrogel composite. The new material, a potential candidate as eco-friendly adsorbent, can rapidly remove MB dye from industrial sewage. What's more, the comprehensive utilization of waste corn stalks was enriched. This action can not only prevent atmospheric contamination from abandoned corn stalks incinerating, but water pollution deriving from organic dyes discharged into water as well. We detailedly investigated the effect of initial MB dye concentration, the dosage of absorbent, agitation rate, contact time, and temperature on the adsorption capacity of the hydrogel composite, using a batch adsorption technique. By virtue of biomicroscopy, the fine structure of the interior of the absorbent after swelling was clearly presented. It allowed for an effective and comprehensive analysis of the relationship between the microstructures and properties of the material [31]. Further, for relatively thorough exploring the mechanisms of MB dye adsorption, the kinetic and diffusion models by the adsorption experimental data were established. Our novel hydrogel composites having extraordinary functions will be engaged in the remediation of industrial effluents.
2. Experimental 2.1. Materials Waste corn stalks were obtained from the Anda farmland (Anda, China). Acrylic acid (AA), Acrylamide (AM), and ethyl alcohol were purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). Chloroacetic acid was supplied by Tianjin Yaohua Chemical Factory (Tianjin, China). N,N'-methylene-bis-acrylamide (MBA) and sodium hydroxide (NaOH) were obtained from Shanghai Jianglai Biological Technology Co., Ltd. (Shanghai, China). Potassium persulfate (KPS) and Methylene blue (MB) dye used for the dye adsorption study were purchased from Huadong Reagent Factory (Shenyang, China). Organic montmorillonite (OMMT) was obtained from Zhejiang Fenghong New materials Co., Ltd. (Zhejiang, China). Nitrogen gas supplied by Daqing Xuelong Gas Co. (Heilongjiang, China) was high purity nitrogen. The others were of analytical grade. 2.2. Pretreatment of corn stalk The washed corn stalks were dried in a 60 ℃ vacuum drying oven for 6 h, smashed, sifted through a 100-mesh sieve. A weight quantity of corn stalks and 15 wt% NaOH solution were charged into a 500 mL three-necked flask. This mixture was placed into water bath kept at 55 ℃ for 2.5 h with mechanical stirring. The samples named as purified corn stalks (PCS) were washed several times with deionized water and then dried at 60 ℃ to a constant weight. Weighted x g PCS, 10x mL ethyl alcohol, and 10 wt% NaOH solution were added into a 500 ml three-necked flask and stirred continuously at 35 ℃ for 2 h. Then 12x g 30 wt% chloroacetic acid-ethanol solution was poured into the reactor at 75 ℃ for 2.5 h. The resulting product (PTCS) was washed thoroughly with deionized water for several times and finally dried at 60 ℃ to a constant weight. 4
2.3. Preparation of the PTCS/OMMT hydrogel composite The PTCS/OMMT hydrogel composite was fabricated by aqueous solution polymerization in a 250 ml three-necked reactor equipped with a mechanical stirrer. 20 wt% NaOH was used as a neutralizing agent to neutralize AA in the ice bath with the neutralization degree of 70%. PTCS, neutralized AA and AM with the mass ratio of 1:5:2 along with K2S2O8 as the initiator were evenly mixed, then they were added into the reactor which was placed into ultrasonic (40 KHz) bath kept at 60 ℃, under N2 protection for the removal of dissolved O2 from the system. An hour later, the required amount of OMMT and MBA as the crosslinker were also added into the system. This reaction wasn’t accomplished until emerging the "climbing-bar" phenomenon. Finally, the target products were dried in a 60 ℃ vacuum drying oven for 12 h, smashed, sifted through a 30-mesh and 50-mesh sieves to store samples whose particle size was 0.3~0.6 mm. The polymerization mechanism is in Scheme 1. 2.4. Characterization of hydrogel 2.4.1. Fourier transform infrared Spectroscopy (FTIR) FTIR spectra of samples were recorded on a Bruker-TENSOR27 infrared spectrometer (Bruker, Germany), in the range of 4000 cm1 and 450 cm1. The desired tablets were prepared by mixing powder of the dry samples and analytical grade KBr. 2.4.2. Field Emission Scanning Electron Microscope (FESEM) Prior to observation, the hydrogel composites were swollen to equilibrium in distilled water at room temperature, then they were withdrawn from water and dried in a vacuum drying oven at 60 ℃ for 24 h. The dry samples were coated with a thin layer of gold to enhance conductivity. The morphological variation of the samples was obtained on a Quanta FEG 450 FESEM (FEI, America). 2.4.3. Biomicroscope analysis A certain amount of hydrogel composites were immersed into distilled water, then they were placed in the sealing bags to maintain for 24 h when the samples absorbed water up to 80 times of their weight, so that the samples presented the state of swelling balance inside and outside. The slices of hydrogels cut after swelling were observed and photographed by Zeiss Axio Scope A1 pol type polarizing microscope (Zeiss, Germany). 2.4. Adsorption studies A stock solution of MB dye (1 g/L) was prepared by dissolving an appropriate amount of dye in l L volumetric flask; the working solution of MB dye was obtained by diluting the stock solution to a desired concentration of 10, 20, 30, 40, and 50 mg/L. Batch adsorption experiments were carried out to investigate the effect of initial dye concentration, the dosage of hydrogel, stirring speed, contact time and temperature on the adsorption of MB dye. A desired time intervals, a supernatant aliquot was collected and analyzed at 665 nm using a UV-1700 spectrophotometer to monitor the adsorption state. The typical fluorescence spectrum and the successive variation were 5
described in Fig.1. The concentrations of MB solution before and after adsorption were determined using a linear regression equation (R2=0.995) by plotting a calibration curve. The amount of MB dye adsorbed by the samples at equilibrium (Qe, mg/g) and at time t (Qt, mg/g) was calculated according to the following equations ((1) and (2)). All adsorption experiments were done in triplicate and the average was used in data analysis. Qe =
(C0 - Ce ) ×V m
(1)
Qt =
(C0 - Ct ) ×V m
(2)
R（%）=
(C0 - Ct ) ×100 C0
(3)
3. Result and discussion 3.1. Fourier transform infrared Spectroscopy (FTIR) analysis FTIR spectra of PTCS, OMMT, and hydrogel composite were quantitatively studied as presented in Fig.2. In the spectra of PTCS and hydrogel, the —OH functional group was observed to overlap in the region 3450~3460 cm1, the peaks at 2889 cm1 and 2921 cm1 was ascribed to the symmetric and asymmetrical stretching vibration of C—H, respectively, further verifying that the characteristic peaks of corn stalk were retained in the hydrogel composite [32]. Band resulting from C=O of —CH2COONa stretching was observed at 1610 cm1, revealing that the carboxymethylation reaction [33]. In addition, the adsorption peak located at 607 cm1 in the spectrum of PTCS is associated with the glucose units’ circular structure [15]. The band at 1039 cm1 was attributed to the in- plane Si—O—Si stretching vibration, and the 519 cm1 and 467 cm1 were assigned to the Si—O bending vibration. The peak at around 627 cm1 was due to bending vibration of Al—O group. The broad absorption at 3445 cm1 was related to the adsorbed water in the interlayer [34,35]. Characteristic changes were depicted in the spectrum of hydrogel composite. The adsorption peak at 1610 cm1 became larger in comparison to the PTCS’s. The new band at 1407 cm1 was ascribed to the C—N stretching vibration [36]. The peaks of Si—O and Al—O were shifted to a lower frequency and their absorption intensity declined. What's more, it was noteworthy that the peaks at 519 cm1 and 467 cm1 derived from the blending vibration of Si—O group completely disappeared, which demonstrated that polymer chains successfully intercalated into the OMMT layers [37]. The adsorption peak located at 607 cm1 almost disappeared as well. From the above discussion, it confirmed a clear incorporation functional monomers and organic clay into modified corn stalks. 3.2. Field Emission Scanning Electron Microscope (FESEM) analysis The FESEM micrographs of PTCS/OMMT hydrogel composite with different magnifications were described in Fig.3. The surface of hydrogel composite distributing of a large number of pores was coarse. These pores were related to the transport of water molecules into the functional materials [38]. In the magnified images (Fig.3b and c), the wrinkles and holes were able to be 6
observed clearly, furthermore, the aggregate of OMMT was not formed, which fully indicated that the exfoliation of OMMT took place in the polymerization process [24]. The delaminated layers of organic clay were uniformly distributed in a continuous polymer matrix. The new materials with extraordinary performance swelling and adsorption should be associated with their structures. 3.3. Biomicroscope analysis Biomicroscopy is applied to effectively observe the fine microstructure of hydrogel composites. The biomicroscope images of samples after swelling with different magnifications were shown in Fig. 4. The network skeletons and interconnected porous structures possessing the stereoscopic effect were clearly presented. These network backbones kept a quite tight arrangement. With increasing the magnification, interestingly, a fraction of free filaments which interweaved with each other to form networks were uniformly presented between the network skeletons and the pore channels, we speculated that the excessive monomers’ homo/copolymers penetrated into networks [15]. This observation method is convenient and efficient to explore the fine microstructures of target products. Such structures were verified that they can promote the rapid penetration of the water or dye molecules [31]. 3.4. MB dye adsorption 3.4.1. Effect of initial dye concentration As shown in Fig. 5a, different initial dye concentrations have an influence on adsorption of MB dye. And other experimental conditions were fixed simultaneously such as the dosage of hydrogel (0.20 g/L), stirring speed (100 r/min), contact time (240 min) and temperature (30 ℃). It was clear from the figure that the dye removal showed noticeable reduction as initial dye concentration from 10 to 50 mg/L, that it was 98.02% when using initial dye concentration of 10 mg/L and decreased to 53.21% by increasing the initial dye concentration to 50 mg/L. Strong electrostatic interaction between functional monomers and dye molecules resulted in high removal percent [9]. We found that the adsorption capacity was weaker at higher dye concentration. It was chiefly because the adsorption sites on the surface had been early reached the saturation state when hydrogel was immersed into the dye at high concentration. Even if the dye concentration was increased, the adsorption capacity of the material no longer increased [39]. 3.4.2. Effect of dosage To investigate the effect of hydrogel composite dosage on removal of MB dye from aqueous solution, the adsorption experiments were carried out using adsorbent dose ranging from 0.10 g/L to 0.40 g/L, other conditions were fixed simultaneously such as initial dye concentration (10 mg/L), stirring speed (100 r/min), contact time (240 min) and temperature (30 ℃). As shown in Fig. 5b, when the dosage increased from 0.10 g/L to 0.20 g/L, the dye removal percent was markedly improved from 75.68% to 98.02%. However, no significant increase in the percentage adsorption was observed with further increase in the adsorbent dose. Hence, 0.20 g/L hydrogel dosage was chosen for subsequent experiments. Similar observation was also made by E.S. 7
Abdel-Halim [40] for the effect of adsorbent dose on the adsorption of Direct Red 81. We all consider two reasons are responsible for these variations. First, it was attributed to the greater availability of the exchangeable sites of the hydrogel composite; second, the complete exhaustion of the dye upon using 0.20 g/L adsorbent dose [40]. 3.4.3. Effect of stirring speed Effect of stirring speed on MB dye adsorption for feed dye concentration of 10, 20, and 40 mg/L was depicted in Fig. 5c. It is clear from the data given in these curves that, the removal percent had no evident variation by changing the stirring speed at low feed concentration (10 mg/L). Nevertheless, the adsorption efficiency was distinctly improved as increasing stirring speed when the material was immersed into the dye solution having concentration 20 mg/L. Surprisingly, above tendency was more significant at high feed concentration (40 mg/L). We found that the dye removal percent was observably improved from 54.78% to 90.64% by varying the stirring speed in the range of 50 r/min~200 r/min. It disclosed that the effect of stirring speed on dye adsorption for high initial dye concentration was greater than low feed concentration. The high stirring speed had the ability to accelerate the removal for MB dye with a higher concentration. 3.4.4. Effect of contact time Fig. 5d presents the enhancement in the amount of MB dye adsorbed by prolonging contact time. The adsorption process was carried out using initial dye concentration of 10 mg/L, adsorbent dose of 0.20 g/L, stirring speed of 100 r/min together with temperature (30 ℃). Before 3 min, the adsorption capacity of hydrogel only was 11.69 mg/g, while it can reach as high as to 46.55 mg/g after 60 min. Subsequently, the amount of MB dye adsorbed slightly increased with lower rate until reaching the equilibrium status (Qe=49.01 mg/g) which was established between the dye molecule of MB dye solution and another dye molecule of hydrogel surface adsorbed. 3.4.5. Effect of temperature To explore the effect of temperature on percent dye removal, twenty-one adsorption trials were carried out under different temperatures conditions, ranging from 30 ℃ to 50 ℃. As shown in Fig. 5e, the removal percentage at the equilibrium status was 98.02%, 98.52%, and 99.70%, respectively, by raising the adsorption temperature from 30 ℃ to 50 ℃, which demonstrated that the temperature had little influence on the adsorption capacity at the equilibrium status. Before reaching the equilibrium state (t = 15 min), the dye removal percent was signally improved from 62.61% to 75.86% by raising the temperature in the range of 30 ℃~50 ℃. Appropriately increasing the temperature had capacity of reinforcing adsorption effect and shortening the time to reach the equilibrium state. The higher temperature can accelerate the mobility of dye ions and swelling property of hydrogel composite, further increase the porosity of absorbent [41]. 3.5. Adsorption kinetics and mechanism 3.5.1. Adsorption kinetics The kinetic adsorption experiments were carried out using adsorbent dose of 0.20 g/L, initial 8
dye concentration (10 mg/L), stirring speed (100 r/min), contact time (240 min) and temperature (30 ℃). Two kinetic models, the pseudo-first-order [42] and pseudo-second-order [43] were considered to evaluate the adsorption kinetics process. The kinetic model of the pseudo-first-order is given by: dQ k1 (Qe Qt ) dt
(4)
A linear form of the equation can be obtained by integrating above equation: ln (Qe－Qt) = ln Qe－k1 t
(5)
The kinetic model of the pseudo-second-order can be represented by: dQ k2 (Qe Qt ) 2 dt
(6)
The liner form of interageted the equation can be expressed as: t 1 1 t 2 Qt k2Qe Qe
(7)
According to adsorption experimental data, the kinetic models of pseudo-first-order and pseudo-second-order were established as described in Fig. 6. The statistical parameters for these adsorptions were summarized in Table 1. The best fitting of the adsorption kinetic models were determined from the correlation coefficients (R2). We found that the pseudo-second-order model had a higher value of R2 (0.999) than the other model (0.933), close to unity. The maximum adsorption capacity was 52.63 mg/g from the theoretical calculation, which closed to the actual value (Qe=49.01 mg/g). So it can be established that the adsorption of MB dye on the hydrogel composite followed the pseudo-second-order kinetic model, which confirmed that the adsorption should be a chemical process. 3.5.2. Adsorption mechanism Generally speaking, adsorption is a multistep process and its overall rate is controlled by the slowest step, maybe either the intraparticle mass transfer rate or the liquid-phase mass transfer rate. Hence, the adsorption kinetic experimental data were applied to build the intraparticle diffusion model and the liquid film diffusion model, so that we can ascertain the rate controlling step [18]. Their expressions can be given as: Qt = ki t0.5 + Ci
(8)
－ln (1－F) = kfd t
(9)
The plots of two models for the adsorption of MB dye at different times are shown in Fig. 7, and some formula parameters are listed in Table 1. It can be clearly seen that the intraparticle diffusion process was composed of three stages. First, MB dye molecules were transferred from dye solution towards the external surface of hydrogel; second, diffusion of the dye into the pores of hydrogel; third, the equilibrium status of dye adsorption [44]. Fig. 7a presents that the intraparticle diffusion plot owned an intercept, not passed through the origin, which illustrated the intraparticle diffusion in the adsorption process was not sole rate-limiting step. In addition, the 9
liquid-film diffusion plot did not pass through the origin as well (Fig. 7b), which indicated that the liquid-film diffusion is not the singular rate-determining step for the adsorption of MB dye onto the hydrogel composite. To sum up, the adsorption process was controlled not only by intraparticle diffusion, but also by film diffusion. The adsorption mechanism was vividly depicted in Scheme 2. 3.9. Comparison with reported work The adsorption capacity by the present PTCS/OMMT hydrogel composite is compared with other adsorbents in Table 2. From the data given in Table 2, it was observed that our present material possessed the capacity of much higher adsorption than some reported work. Or rather, the greatest contribution of this study was not only to improve the removal of organic dye from sewage water, but the comprehensive utilization of waste corn stalks as well.
4. Conclusion The novel environment-friendly material was successfully fabricated using farmland waste as the main ingredient. FTIR confirmed a clear incorporation functional monomers and organic clay into modified corn stalks. The delaminated layers of OMMT were uniformly distributed in a continuous polymer matrix, as shown in the FESEM images. In addition, the network skeletons and interconnected porous structures were also clearly presented in the biomicroscope images, kept a quite tight arrangement. Such microstructure possessing the stereoscopic effect was different from the FESEM images. To investigate some factors which were expected to affect the adsorption process, a batch of adsorption experiments were carried out. The results show that 99.70% MB dye was removed with a PTCS/OMMT hydrogel composite dose of 0.2 g/L and temperature of 50 ℃. The adsorption process should be a chemical adsorption and the experimental data fit well with the pseudo-second-order kinetic model, based on the higher R2 value. We found that this adsorption was a multistep process, which controlled not only by intraparticle diffusion, but also by film diffusion, simultaneously. The maximum adsorption capacity of our materials was 49.01 mg/g, which is comparable to other reported adsorbents. This functional material has the potential to be used as a high-efficiency adsorbent for the remediation of MB dye contaminated water.
Acknowledgements This work was supported by the National Undergraduate Training Programs for Innovation and Entrepreneurship (No.201310220014). The authors would like to thank the very useful suggestion from the editors and reviewers.
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14
1.2
665
Absorbancy
1.0 0.8 0.6 0.4 0.2 0.0 100
200
300
400
500
600
700
800
900
Wavelength (nm) Fig.1. The typical fluorescence spectrum and the successive variation of MB solution.
Si-O
c 2921
b Al-O 467
519
a
C=O 2889
C-N
O-H
4500
3600
2700
607
1800
1
900
0
Wavenumber/cm
Fig.2. FTIR spectra of PTCS (a), OMMT (b), and PTCS/OMMT hydrogel composite (c).
(a)
(b)
(c)
Fig.3. SEM images of the PTCS/OMMT hydrogel composite with different magnifications. 500× (a), 4000× (b), and 10000× (c).
15
(a)
(b)
(c)
Fig.4. The biomicroscope images of the PTCS/OMMT hydrogel composite with different magnifications. 4000× (a), 8000× (b), and 16000× (c).
16
100
100
90
95
70
Dye removal (%)
Dye removal (%)
80
60 50 40 30 20
90 85 80 75
10 0 10
20
30 C0 (mg/L)
40
70 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 Dosage of absorbent（ g/L）
50
(a)
(b) 60
90
48
80
Qt (mg/g)
Dye removal (%)
100
70 10mg/L 20mg/L 40mg/L
60 50 20
40
60
36 24 12 0
80 100 120 140 160 180 200 220 Stirring speed (r/min)
(c)
3
6
15
30 60 t (min)
90
120
240
(d)
105 100 Dye removal (%)
95 90 85 80 75 70
30 40 50
65 60 55
0
50
100
150 200 t (min)
250
℃
℃
℃
300
350
(e) Fig.5. Effect of different adsorption conditions on the adsorption of MB dye. Initial dye concentration (a), dosage of absorbent (b), stirring speed (c), contact time (d), and temperature (e).
17
4
5 3
4 3
1 t/Qt
ln (Qe-Qt)
2
0
2
-1
1
-2
0 -3
0
50
100 150 t (min)
200
250
0
(a)
50
100 150 t (min)
200
250
(b)
Fig.6. Plots for the pseudo-first-order model (a) and pseudo-second-order model (b) for the adsorption of MB dye onto PTCS/OMMT hydrogel composite.
7
60
6
50
-ln (1-F)
Qt (mg/g)
5
40 30 20
3 2
10 0
4
1
0
2
4
6
8
1/2
10 1/2
12
14
16
0
18
t (min )
(a)
0
50
100
150 t（ min）
200
250
(b)
Fig.7. Plots for the intraparticle diffusion model (a) and liquid film diffusion model (b) for the adsorption of MB dye onto PTCS/OMMT hydrogel composite.
18
Scheme 1 The polymerization mechanism of the PTCS/OMMT hydrogel composite.
Scheme 2 The adsorption mechanism for the adsorption of MB dye onto PTCS/OMMT hydrogel composite.
19
Table 1 Kinetic parameters for the adsorption of MB dye onto PTCS/OMMT hydrogel composite. Pseudo-first-order model
Pseudo-second-order model
Qe exp (mg·g1)
49.01
k1 （min1） 2.47×102
Qe cal R2
0.9334
k2
(g·mg1)
(g·mg1·min1)
21.91
3.61×104
Intraparticle diffusion model Diffusion
ki
models
(mg·g1·min0.5) 7.99
Qe cal R2
(g·mg1)
0.9997
Liquid-film diffusion model kfd
R2
Ci
0.9789
0.65
(min1) 2.47×102
R2 0.9334
Table 2 Comparison of present work with reported data. Adsorbent
Adsorbate
Q (mg/g)
Refs.
Posidonia oceanica (L.) fibres
MB
5.56
[45]
Aspergillus niger
MB
18.54
[46]
Cereal chaff
MB
20.30
[47]
PNIPAAm
MB
8.95
[48]
P(NIPAAm/IA)
MB
17.67
[48]
P(NIPAAm/IA/Pumice)
MB
22.62
[48]
poly(AMPS-co-IA)
MB
24.10
[49]
Polyacrylamide/polyacrylate/gum Arabic
MB
48.00
[50]
PTCS/OMMT hydrogel composite
MB
49.01
Present work
20
52.63
S0169-4332(17)31728-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.072 APSUSC 36272
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-3-2017 23-5-2017 6-6-2017
Please cite this article as: Dongzhuo Ma, Baodong Zhu, Bo Cao, Jian Wang, Jianwei Zhang, Fabrication of the novel hydrogel based on waste corn stalk for removal of methylene blue dye from aqueous solution, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.072 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.
Fabrication of the novel hydrogel based on waste corn stalk for removal of methylene blue dye from aqueous solution Dongzhuo Ma1, Baodong Zhu1,∗, Bo Cao2, Jian Wang1, Jianwei Zhang1 1
Provincial Key Laboratory Oil and Gas Chemical Technology, College of Chemistry and
Chemical Engineering, Northeast Petroleum University, Daqing, Heilongjiang 163318, China 2
Daqing Oil Field Co., Daqing, Heilongjiang 163453, China
∗ Corresponding author. Tel.: +86 13836884622. E-mail address: [email protected] (B.D. Zhu).
Graphical abstract:
Highlights:
The novel hydrogel based on waste corn stalk was successfully fabricated by aqueous solution polymerization technique.
The fine microstructures of the material were clearly observed by Biomicroscope, which was different from the FESEM images.
99.70% MB dye was rapidly removed with the novel hydrogel composite.
The utilization of waste corn stalks was enriched.
It will be engaged in the remediation of industrial effluents.
Abstract The novel hydrogel based on waste corn stalk was synthetized by aqueous solution polymerization technique with functional monomers in the presence of organic montmorillonite (OMMT) under ultrasonic. In this study, batch adsorption experiments were carried out to research 1
the effect of initial dye concentration, the dosage of hydrogel, stirring speed, contact time and temperature on the adsorption of methylene blue (MB) dye. The adsorption process was best described by the pseudo-second-order kinetic model, which confirmed that it should be a chemical process. Furthermore, we ascertained the rate controlling step by establishing the intraparticle diffusion model and the liquid film diffusion model. The adsorption and synthesis mechanisms were vividly depicted in our work as well. Structural and morphological characterizations by virtue of FTIR, FESEM, and Biomicroscope supported the relationship between the adsorption performance and material’s microstructure. This research is a valuable contribution for the environmental protection, which not only converts waste corn stalks into functional materials, but improves the removal of organic dye from sewage water.
Key words: Corn stalk; Clay; Hydrogel; Adsorption; Kinetics
1. Introduction Corn stalk whose main chemical components contain abundant cellulose, hemicellulose and lignin is one of the by-products of grain crops [1]. So far the utilization of waste corn stalk is not high, except for a limited comprehensive utilization as feed, fertilizer, industrial raw materials, fuel, and so forth [2]. Others are mostly incinerated, which not only gives rise to the huge waste of biomass resources, but the serious environment pollutions as well. Cellulose which belongs to a linear and extended polysaccharide stands out as one of the most significant natural polymers [3]. If the corn stalks which reserve 40-60% natural cellulose are applied to the fabrication of cellulose-based hydrogels, the farmland waste will play a crucial role on the field of environmental protection. Those polysaccharide-based biodegradable materials possess attractive advantages like low cost, easily available, nontoxicity, renewability, biocompatibility, environment -ally friendly natures, and so on [4]. However, their applications have been limited on account of the poor mechanical property and low adsorption capacity. To address the problem, many researchers have kept trying to modify these polymers by pretreatment and functionalization [5]. Grafting copolymerization becomes a convenient and efficient approach for chemical modification of cellulose that for instance introducing some special monomers with functional groups such as hydroxyl group, carboxyl group, amino group, sulfo group, onto the backbone of cellulose [6,7]. Copolymerizing different types of monomers and polysaccharides will be a versatile option for joining the excellent properties of both [8], which can eliminate the weakness from the performance of the polysaccharide-based hydrogel [9]. The hydrogels own three-dimensional hydrophilic networks of both natural and synthetic polymers via chemically or physically crosslinking [10]. They are the classic representative of soft materials [11], which has gained extensive attention as biological, agriculture and eco-friendly function materials [2,12-14]. In addition, they are insoluble and possess the ability to absorb and retain a larger volume of water 2
by means of networks and pore structures without losing their physical dimension structure [15,16]. Nowadays, it has been reported that a great deal of industries for instance cosmetic, textile, printing, plastics, paper production, leather, food industries often use dyes in their production processes [17]. Nevertheless, these dye effluents are discharged into the environment without any water treatment. Most of dyes are toxic, carcinogenic and mutagenic, even at low concentrations, which are harmful to the human health [18]. Furthermore, these wastes lower the water quality, which represent risk to aquatic life. The bulk of photosynthesizing species are also under threat due to the restriction in light penetration in dye-contaminated water. Methylene blue (MB), a cationic dye, officially named as 3,7-bis(dimethylamino)-phenothiazin-5-ium chloride, is considered more toxic than those anionic dyes [19]. It can easily permeate into the cells by interacting with negatively charged surface of cell membrane. So, rapid and highly effective removal of MB dye from industrial effluents has been a challenging research project [20]. Up to now, various methods for removal of MB dye from aqueous solution including membrane separation, oxidation or ozone treatment, electrolysis, ion-exchange, chemical precipitation, photocatalytic degradation, adsorption, and the like [21]. Among these technologies, the adsorption has been found to be a kind of flexible and economical approach on the removal of different synthetic dyes [22]. Clays become one of popular materials as adsorbents for dyes currently, like montmorillonite (MMT) consists of layers of two tetrahedral silica sheets sandwiching one octahedral alumina sheet [23]. MMT has been widely used as an adsorbent due to its large specific surface and robust cation-exchange ability [24]. In order to improve the compatibility between MMT and polymer matrix, the chemical modification of hydrophilic silicates is urgently required, so the organo-clays are born at the right moment [25]. In previous studies, natural polymer/clay hydrogel composites had been paid great attention to use them as absorbents for removal of contaminants [26]. Yi et al. [27] successfully prepared a novel hybrid hydrogel using sodium humate, polyacrylamide, and hydrophilic laponite. The adsorption and desorption of MB experiments show that it had the good adsorption capacity and the maximum absorption concentration of MB was 800 mg/L. Shirsath et al. [28] produced the hydrogel composite by incorporation of kaoline clay to remove Brilliant Green dye. It had been observed that the maximum dye removal by PAA-K hydrogel was achieved for the initial dye concentration of 30 mg/L at a temperature of 35 ℃ and pH=7, further, it confirmed the contribution of clay minerals. In addition, the bentonite filler was also introduced into the hydrogel, this nano-clay filled composite hydrogel can efficiently finish the removal of some cationic dyes or metal ions from sewage [29]. Many researchers are devoting themselves to develop the new functional materials for wastewater treatment [30]. Surprisingly, there is less report about wastewater treatment using waste corn stalks. In our study, we selected the corn stalk as an ingredient which grafted diffident functional monomers 3
such as acrylic acid (AA) and acrylamide (AM). Organic montmorillonite (OMMT) was also chosen for the fabrication of the novel hydrogel composite. The new material, a potential candidate as eco-friendly adsorbent, can rapidly remove MB dye from industrial sewage. What's more, the comprehensive utilization of waste corn stalks was enriched. This action can not only prevent atmospheric contamination from abandoned corn stalks incinerating, but water pollution deriving from organic dyes discharged into water as well. We detailedly investigated the effect of initial MB dye concentration, the dosage of absorbent, agitation rate, contact time, and temperature on the adsorption capacity of the hydrogel composite, using a batch adsorption technique. By virtue of biomicroscopy, the fine structure of the interior of the absorbent after swelling was clearly presented. It allowed for an effective and comprehensive analysis of the relationship between the microstructures and properties of the material [31]. Further, for relatively thorough exploring the mechanisms of MB dye adsorption, the kinetic and diffusion models by the adsorption experimental data were established. Our novel hydrogel composites having extraordinary functions will be engaged in the remediation of industrial effluents.
2. Experimental 2.1. Materials Waste corn stalks were obtained from the Anda farmland (Anda, China). Acrylic acid (AA), Acrylamide (AM), and ethyl alcohol were purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). Chloroacetic acid was supplied by Tianjin Yaohua Chemical Factory (Tianjin, China). N,N'-methylene-bis-acrylamide (MBA) and sodium hydroxide (NaOH) were obtained from Shanghai Jianglai Biological Technology Co., Ltd. (Shanghai, China). Potassium persulfate (KPS) and Methylene blue (MB) dye used for the dye adsorption study were purchased from Huadong Reagent Factory (Shenyang, China). Organic montmorillonite (OMMT) was obtained from Zhejiang Fenghong New materials Co., Ltd. (Zhejiang, China). Nitrogen gas supplied by Daqing Xuelong Gas Co. (Heilongjiang, China) was high purity nitrogen. The others were of analytical grade. 2.2. Pretreatment of corn stalk The washed corn stalks were dried in a 60 ℃ vacuum drying oven for 6 h, smashed, sifted through a 100-mesh sieve. A weight quantity of corn stalks and 15 wt% NaOH solution were charged into a 500 mL three-necked flask. This mixture was placed into water bath kept at 55 ℃ for 2.5 h with mechanical stirring. The samples named as purified corn stalks (PCS) were washed several times with deionized water and then dried at 60 ℃ to a constant weight. Weighted x g PCS, 10x mL ethyl alcohol, and 10 wt% NaOH solution were added into a 500 ml three-necked flask and stirred continuously at 35 ℃ for 2 h. Then 12x g 30 wt% chloroacetic acid-ethanol solution was poured into the reactor at 75 ℃ for 2.5 h. The resulting product (PTCS) was washed thoroughly with deionized water for several times and finally dried at 60 ℃ to a constant weight. 4
2.3. Preparation of the PTCS/OMMT hydrogel composite The PTCS/OMMT hydrogel composite was fabricated by aqueous solution polymerization in a 250 ml three-necked reactor equipped with a mechanical stirrer. 20 wt% NaOH was used as a neutralizing agent to neutralize AA in the ice bath with the neutralization degree of 70%. PTCS, neutralized AA and AM with the mass ratio of 1:5:2 along with K2S2O8 as the initiator were evenly mixed, then they were added into the reactor which was placed into ultrasonic (40 KHz) bath kept at 60 ℃, under N2 protection for the removal of dissolved O2 from the system. An hour later, the required amount of OMMT and MBA as the crosslinker were also added into the system. This reaction wasn’t accomplished until emerging the "climbing-bar" phenomenon. Finally, the target products were dried in a 60 ℃ vacuum drying oven for 12 h, smashed, sifted through a 30-mesh and 50-mesh sieves to store samples whose particle size was 0.3~0.6 mm. The polymerization mechanism is in Scheme 1. 2.4. Characterization of hydrogel 2.4.1. Fourier transform infrared Spectroscopy (FTIR) FTIR spectra of samples were recorded on a Bruker-TENSOR27 infrared spectrometer (Bruker, Germany), in the range of 4000 cm1 and 450 cm1. The desired tablets were prepared by mixing powder of the dry samples and analytical grade KBr. 2.4.2. Field Emission Scanning Electron Microscope (FESEM) Prior to observation, the hydrogel composites were swollen to equilibrium in distilled water at room temperature, then they were withdrawn from water and dried in a vacuum drying oven at 60 ℃ for 24 h. The dry samples were coated with a thin layer of gold to enhance conductivity. The morphological variation of the samples was obtained on a Quanta FEG 450 FESEM (FEI, America). 2.4.3. Biomicroscope analysis A certain amount of hydrogel composites were immersed into distilled water, then they were placed in the sealing bags to maintain for 24 h when the samples absorbed water up to 80 times of their weight, so that the samples presented the state of swelling balance inside and outside. The slices of hydrogels cut after swelling were observed and photographed by Zeiss Axio Scope A1 pol type polarizing microscope (Zeiss, Germany). 2.4. Adsorption studies A stock solution of MB dye (1 g/L) was prepared by dissolving an appropriate amount of dye in l L volumetric flask; the working solution of MB dye was obtained by diluting the stock solution to a desired concentration of 10, 20, 30, 40, and 50 mg/L. Batch adsorption experiments were carried out to investigate the effect of initial dye concentration, the dosage of hydrogel, stirring speed, contact time and temperature on the adsorption of MB dye. A desired time intervals, a supernatant aliquot was collected and analyzed at 665 nm using a UV-1700 spectrophotometer to monitor the adsorption state. The typical fluorescence spectrum and the successive variation were 5
described in Fig.1. The concentrations of MB solution before and after adsorption were determined using a linear regression equation (R2=0.995) by plotting a calibration curve. The amount of MB dye adsorbed by the samples at equilibrium (Qe, mg/g) and at time t (Qt, mg/g) was calculated according to the following equations ((1) and (2)). All adsorption experiments were done in triplicate and the average was used in data analysis. Qe =
(C0 - Ce ) ×V m
(1)
Qt =
(C0 - Ct ) ×V m
(2)
R（%）=
(C0 - Ct ) ×100 C0
(3)
3. Result and discussion 3.1. Fourier transform infrared Spectroscopy (FTIR) analysis FTIR spectra of PTCS, OMMT, and hydrogel composite were quantitatively studied as presented in Fig.2. In the spectra of PTCS and hydrogel, the —OH functional group was observed to overlap in the region 3450~3460 cm1, the peaks at 2889 cm1 and 2921 cm1 was ascribed to the symmetric and asymmetrical stretching vibration of C—H, respectively, further verifying that the characteristic peaks of corn stalk were retained in the hydrogel composite [32]. Band resulting from C=O of —CH2COONa stretching was observed at 1610 cm1, revealing that the carboxymethylation reaction [33]. In addition, the adsorption peak located at 607 cm1 in the spectrum of PTCS is associated with the glucose units’ circular structure [15]. The band at 1039 cm1 was attributed to the in- plane Si—O—Si stretching vibration, and the 519 cm1 and 467 cm1 were assigned to the Si—O bending vibration. The peak at around 627 cm1 was due to bending vibration of Al—O group. The broad absorption at 3445 cm1 was related to the adsorbed water in the interlayer [34,35]. Characteristic changes were depicted in the spectrum of hydrogel composite. The adsorption peak at 1610 cm1 became larger in comparison to the PTCS’s. The new band at 1407 cm1 was ascribed to the C—N stretching vibration [36]. The peaks of Si—O and Al—O were shifted to a lower frequency and their absorption intensity declined. What's more, it was noteworthy that the peaks at 519 cm1 and 467 cm1 derived from the blending vibration of Si—O group completely disappeared, which demonstrated that polymer chains successfully intercalated into the OMMT layers [37]. The adsorption peak located at 607 cm1 almost disappeared as well. From the above discussion, it confirmed a clear incorporation functional monomers and organic clay into modified corn stalks. 3.2. Field Emission Scanning Electron Microscope (FESEM) analysis The FESEM micrographs of PTCS/OMMT hydrogel composite with different magnifications were described in Fig.3. The surface of hydrogel composite distributing of a large number of pores was coarse. These pores were related to the transport of water molecules into the functional materials [38]. In the magnified images (Fig.3b and c), the wrinkles and holes were able to be 6
observed clearly, furthermore, the aggregate of OMMT was not formed, which fully indicated that the exfoliation of OMMT took place in the polymerization process [24]. The delaminated layers of organic clay were uniformly distributed in a continuous polymer matrix. The new materials with extraordinary performance swelling and adsorption should be associated with their structures. 3.3. Biomicroscope analysis Biomicroscopy is applied to effectively observe the fine microstructure of hydrogel composites. The biomicroscope images of samples after swelling with different magnifications were shown in Fig. 4. The network skeletons and interconnected porous structures possessing the stereoscopic effect were clearly presented. These network backbones kept a quite tight arrangement. With increasing the magnification, interestingly, a fraction of free filaments which interweaved with each other to form networks were uniformly presented between the network skeletons and the pore channels, we speculated that the excessive monomers’ homo/copolymers penetrated into networks [15]. This observation method is convenient and efficient to explore the fine microstructures of target products. Such structures were verified that they can promote the rapid penetration of the water or dye molecules [31]. 3.4. MB dye adsorption 3.4.1. Effect of initial dye concentration As shown in Fig. 5a, different initial dye concentrations have an influence on adsorption of MB dye. And other experimental conditions were fixed simultaneously such as the dosage of hydrogel (0.20 g/L), stirring speed (100 r/min), contact time (240 min) and temperature (30 ℃). It was clear from the figure that the dye removal showed noticeable reduction as initial dye concentration from 10 to 50 mg/L, that it was 98.02% when using initial dye concentration of 10 mg/L and decreased to 53.21% by increasing the initial dye concentration to 50 mg/L. Strong electrostatic interaction between functional monomers and dye molecules resulted in high removal percent [9]. We found that the adsorption capacity was weaker at higher dye concentration. It was chiefly because the adsorption sites on the surface had been early reached the saturation state when hydrogel was immersed into the dye at high concentration. Even if the dye concentration was increased, the adsorption capacity of the material no longer increased [39]. 3.4.2. Effect of dosage To investigate the effect of hydrogel composite dosage on removal of MB dye from aqueous solution, the adsorption experiments were carried out using adsorbent dose ranging from 0.10 g/L to 0.40 g/L, other conditions were fixed simultaneously such as initial dye concentration (10 mg/L), stirring speed (100 r/min), contact time (240 min) and temperature (30 ℃). As shown in Fig. 5b, when the dosage increased from 0.10 g/L to 0.20 g/L, the dye removal percent was markedly improved from 75.68% to 98.02%. However, no significant increase in the percentage adsorption was observed with further increase in the adsorbent dose. Hence, 0.20 g/L hydrogel dosage was chosen for subsequent experiments. Similar observation was also made by E.S. 7
Abdel-Halim [40] for the effect of adsorbent dose on the adsorption of Direct Red 81. We all consider two reasons are responsible for these variations. First, it was attributed to the greater availability of the exchangeable sites of the hydrogel composite; second, the complete exhaustion of the dye upon using 0.20 g/L adsorbent dose [40]. 3.4.3. Effect of stirring speed Effect of stirring speed on MB dye adsorption for feed dye concentration of 10, 20, and 40 mg/L was depicted in Fig. 5c. It is clear from the data given in these curves that, the removal percent had no evident variation by changing the stirring speed at low feed concentration (10 mg/L). Nevertheless, the adsorption efficiency was distinctly improved as increasing stirring speed when the material was immersed into the dye solution having concentration 20 mg/L. Surprisingly, above tendency was more significant at high feed concentration (40 mg/L). We found that the dye removal percent was observably improved from 54.78% to 90.64% by varying the stirring speed in the range of 50 r/min~200 r/min. It disclosed that the effect of stirring speed on dye adsorption for high initial dye concentration was greater than low feed concentration. The high stirring speed had the ability to accelerate the removal for MB dye with a higher concentration. 3.4.4. Effect of contact time Fig. 5d presents the enhancement in the amount of MB dye adsorbed by prolonging contact time. The adsorption process was carried out using initial dye concentration of 10 mg/L, adsorbent dose of 0.20 g/L, stirring speed of 100 r/min together with temperature (30 ℃). Before 3 min, the adsorption capacity of hydrogel only was 11.69 mg/g, while it can reach as high as to 46.55 mg/g after 60 min. Subsequently, the amount of MB dye adsorbed slightly increased with lower rate until reaching the equilibrium status (Qe=49.01 mg/g) which was established between the dye molecule of MB dye solution and another dye molecule of hydrogel surface adsorbed. 3.4.5. Effect of temperature To explore the effect of temperature on percent dye removal, twenty-one adsorption trials were carried out under different temperatures conditions, ranging from 30 ℃ to 50 ℃. As shown in Fig. 5e, the removal percentage at the equilibrium status was 98.02%, 98.52%, and 99.70%, respectively, by raising the adsorption temperature from 30 ℃ to 50 ℃, which demonstrated that the temperature had little influence on the adsorption capacity at the equilibrium status. Before reaching the equilibrium state (t = 15 min), the dye removal percent was signally improved from 62.61% to 75.86% by raising the temperature in the range of 30 ℃~50 ℃. Appropriately increasing the temperature had capacity of reinforcing adsorption effect and shortening the time to reach the equilibrium state. The higher temperature can accelerate the mobility of dye ions and swelling property of hydrogel composite, further increase the porosity of absorbent [41]. 3.5. Adsorption kinetics and mechanism 3.5.1. Adsorption kinetics The kinetic adsorption experiments were carried out using adsorbent dose of 0.20 g/L, initial 8
dye concentration (10 mg/L), stirring speed (100 r/min), contact time (240 min) and temperature (30 ℃). Two kinetic models, the pseudo-first-order [42] and pseudo-second-order [43] were considered to evaluate the adsorption kinetics process. The kinetic model of the pseudo-first-order is given by: dQ k1 (Qe Qt ) dt
(4)
A linear form of the equation can be obtained by integrating above equation: ln (Qe－Qt) = ln Qe－k1 t
(5)
The kinetic model of the pseudo-second-order can be represented by: dQ k2 (Qe Qt ) 2 dt
(6)
The liner form of interageted the equation can be expressed as: t 1 1 t 2 Qt k2Qe Qe
(7)
According to adsorption experimental data, the kinetic models of pseudo-first-order and pseudo-second-order were established as described in Fig. 6. The statistical parameters for these adsorptions were summarized in Table 1. The best fitting of the adsorption kinetic models were determined from the correlation coefficients (R2). We found that the pseudo-second-order model had a higher value of R2 (0.999) than the other model (0.933), close to unity. The maximum adsorption capacity was 52.63 mg/g from the theoretical calculation, which closed to the actual value (Qe=49.01 mg/g). So it can be established that the adsorption of MB dye on the hydrogel composite followed the pseudo-second-order kinetic model, which confirmed that the adsorption should be a chemical process. 3.5.2. Adsorption mechanism Generally speaking, adsorption is a multistep process and its overall rate is controlled by the slowest step, maybe either the intraparticle mass transfer rate or the liquid-phase mass transfer rate. Hence, the adsorption kinetic experimental data were applied to build the intraparticle diffusion model and the liquid film diffusion model, so that we can ascertain the rate controlling step [18]. Their expressions can be given as: Qt = ki t0.5 + Ci
(8)
－ln (1－F) = kfd t
(9)
The plots of two models for the adsorption of MB dye at different times are shown in Fig. 7, and some formula parameters are listed in Table 1. It can be clearly seen that the intraparticle diffusion process was composed of three stages. First, MB dye molecules were transferred from dye solution towards the external surface of hydrogel; second, diffusion of the dye into the pores of hydrogel; third, the equilibrium status of dye adsorption [44]. Fig. 7a presents that the intraparticle diffusion plot owned an intercept, not passed through the origin, which illustrated the intraparticle diffusion in the adsorption process was not sole rate-limiting step. In addition, the 9
liquid-film diffusion plot did not pass through the origin as well (Fig. 7b), which indicated that the liquid-film diffusion is not the singular rate-determining step for the adsorption of MB dye onto the hydrogel composite. To sum up, the adsorption process was controlled not only by intraparticle diffusion, but also by film diffusion. The adsorption mechanism was vividly depicted in Scheme 2. 3.9. Comparison with reported work The adsorption capacity by the present PTCS/OMMT hydrogel composite is compared with other adsorbents in Table 2. From the data given in Table 2, it was observed that our present material possessed the capacity of much higher adsorption than some reported work. Or rather, the greatest contribution of this study was not only to improve the removal of organic dye from sewage water, but the comprehensive utilization of waste corn stalks as well.
4. Conclusion The novel environment-friendly material was successfully fabricated using farmland waste as the main ingredient. FTIR confirmed a clear incorporation functional monomers and organic clay into modified corn stalks. The delaminated layers of OMMT were uniformly distributed in a continuous polymer matrix, as shown in the FESEM images. In addition, the network skeletons and interconnected porous structures were also clearly presented in the biomicroscope images, kept a quite tight arrangement. Such microstructure possessing the stereoscopic effect was different from the FESEM images. To investigate some factors which were expected to affect the adsorption process, a batch of adsorption experiments were carried out. The results show that 99.70% MB dye was removed with a PTCS/OMMT hydrogel composite dose of 0.2 g/L and temperature of 50 ℃. The adsorption process should be a chemical adsorption and the experimental data fit well with the pseudo-second-order kinetic model, based on the higher R2 value. We found that this adsorption was a multistep process, which controlled not only by intraparticle diffusion, but also by film diffusion, simultaneously. The maximum adsorption capacity of our materials was 49.01 mg/g, which is comparable to other reported adsorbents. This functional material has the potential to be used as a high-efficiency adsorbent for the remediation of MB dye contaminated water.
Acknowledgements This work was supported by the National Undergraduate Training Programs for Innovation and Entrepreneurship (No.201310220014). The authors would like to thank the very useful suggestion from the editors and reviewers.
10
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14
1.2
665
Absorbancy
1.0 0.8 0.6 0.4 0.2 0.0 100
200
300
400
500
600
700
800
900
Wavelength (nm) Fig.1. The typical fluorescence spectrum and the successive variation of MB solution.
Si-O
c 2921
b Al-O 467
519
a
C=O 2889
C-N
O-H
4500
3600
2700
607
1800
1
900
0
Wavenumber/cm
Fig.2. FTIR spectra of PTCS (a), OMMT (b), and PTCS/OMMT hydrogel composite (c).
(a)
(b)
(c)
Fig.3. SEM images of the PTCS/OMMT hydrogel composite with different magnifications. 500× (a), 4000× (b), and 10000× (c).
15
(a)
(b)
(c)
Fig.4. The biomicroscope images of the PTCS/OMMT hydrogel composite with different magnifications. 4000× (a), 8000× (b), and 16000× (c).
16
100
100
90
95
70
Dye removal (%)
Dye removal (%)
80
60 50 40 30 20
90 85 80 75
10 0 10
20
30 C0 (mg/L)
40
70 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 Dosage of absorbent（ g/L）
50
(a)
(b) 60
90
48
80
Qt (mg/g)
Dye removal (%)
100
70 10mg/L 20mg/L 40mg/L
60 50 20
40
60
36 24 12 0
80 100 120 140 160 180 200 220 Stirring speed (r/min)
(c)
3
6
15
30 60 t (min)
90
120
240
(d)
105 100 Dye removal (%)
95 90 85 80 75 70
30 40 50
65 60 55
0
50
100
150 200 t (min)
250
℃
℃
℃
300
350
(e) Fig.5. Effect of different adsorption conditions on the adsorption of MB dye. Initial dye concentration (a), dosage of absorbent (b), stirring speed (c), contact time (d), and temperature (e).
17
4
5 3
4 3
1 t/Qt
ln (Qe-Qt)
2
0
2
-1
1
-2
0 -3
0
50
100 150 t (min)
200
250
0
(a)
50
100 150 t (min)
200
250
(b)
Fig.6. Plots for the pseudo-first-order model (a) and pseudo-second-order model (b) for the adsorption of MB dye onto PTCS/OMMT hydrogel composite.
7
60
6
50
-ln (1-F)
Qt (mg/g)
5
40 30 20
3 2
10 0
4
1
0
2
4
6
8
1/2
10 1/2
12
14
16
0
18
t (min )
(a)
0
50
100
150 t（ min）
200
250
(b)
Fig.7. Plots for the intraparticle diffusion model (a) and liquid film diffusion model (b) for the adsorption of MB dye onto PTCS/OMMT hydrogel composite.
18
Scheme 1 The polymerization mechanism of the PTCS/OMMT hydrogel composite.
Scheme 2 The adsorption mechanism for the adsorption of MB dye onto PTCS/OMMT hydrogel composite.
19
Table 1 Kinetic parameters for the adsorption of MB dye onto PTCS/OMMT hydrogel composite. Pseudo-first-order model
Pseudo-second-order model
Qe exp (mg·g1)
49.01
k1 （min1） 2.47×102
Qe cal R2
0.9334
k2
(g·mg1)
(g·mg1·min1)
21.91
3.61×104
Intraparticle diffusion model Diffusion
ki
models
(mg·g1·min0.5) 7.99
Qe cal R2
(g·mg1)
0.9997
Liquid-film diffusion model kfd
R2
Ci
0.9789
0.65
(min1) 2.47×102
R2 0.9334
Table 2 Comparison of present work with reported data. Adsorbent
Adsorbate
Q (mg/g)
Refs.
Posidonia oceanica (L.) fibres
MB
5.56
[45]
Aspergillus niger
MB
18.54
[46]
Cereal chaff
MB
20.30
[47]
PNIPAAm
MB
8.95
[48]
P(NIPAAm/IA)
MB
17.67
[48]
P(NIPAAm/IA/Pumice)
MB
22.62
[48]
poly(AMPS-co-IA)
MB
24.10
[49]
Polyacrylamide/polyacrylate/gum Arabic
MB
48.00
[50]
PTCS/OMMT hydrogel composite
MB
49.01
Present work
20
52.63