Removal of Anionic Dyes from Aqueous Solutions by Cellulose-Based

Article pubs.acs.org/jced

Removal of Anionic Dyes from Aqueous Solutions by Cellulose-Based Adsorbents: Equilibrium, Kinetics, and Thermodynamics Yixi Wang,†,‡ Linyan Zhao,†,‡ Huili Peng,†,‡ Jianning Wu,†,‡ Zhiyong Liu,*,†,‡ and Xuhong Guo†,‡,§ †

School of Chemistry and Chemical Engineering, Shihezi University, Shihezi, Xinjiang 832003, P.R. China Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Key Laboratory of Materials-Oriented Chemical Engineering of Xinjiang Uygur Autonomous Region, Engineering Research Center of Materials-Oriented Chemical Engineering of Xinjiang Bingtuan, Shihezi, Xinjiang 832003, P.R. China § State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P.R. China ‡

ABSTRACT: A novel cellulose (CE)-based adsorbent was synthesized by polyacrylamide grafted quaternized cellulose (PAM-g-QC) to remove anionic dyes. The elemental analysis results showed that there were abundant amino groups on adsorbents and scanning electron microscopy indicated the structure was porousproperties that were a benefit to increase the adsorption ability. The adsorption ability was specifically investigated in different temperatures. Adsorption isotherms including the Langmuir, Freundlich, and Temkin isotherm models were studied at different temperatures (303−323 K), and the maximum theoretical adsorption capacity of the adsorbents was 380.084 mg g−1 for Congo red (CR) and 349.284 mg g−1 for Eriochrome blue SE (EBSE) at 323 K by the Langmuir isotherm model. The adsorption data was fitted to the adsorption kinetics (pseudo-first-order kinetic, pseudo-secondorder kinetic, and intraparticle diffusion models), and the pseudo-second-order kinetic model showed a better fit for the adsorption of CR and EBSE. Thermodynamic parameters (ΔHo, ΔSo, and ΔGo) showed that the adsorption of EBSE and CR was feasible, endothermic, and spontaneous from 303 to 323 K. The adsorption and desorption mechanism of CR and EBSE also was investigated in this study, which verified that electrostatic adsorption played a main role between adsorbents and adsorbates. A comparison with other cellulose-based adsorbents confirmed that the CE/PAM-g-QC adsorbent possessed excellent potential and practical value in dye wastewater treatment.

1. INTRODUCTION Dyes are widely used in the industries of textile, leather, paper, printing, plastics, electroplating, food, and cosmetics, etc.1−3 A large number of dyes are not fully used and are released into the environment, which causes serious pollution. The degradation of dyes is difficult because of their complex organic structures. More seriously, most organic dyes are toxic, nonbiodegradable, mutagenic, and even carcinogenic, which creates serious threats to human health and marine organisms.4 Therefore, the removal of dyes from wastewater is urgent to solve the ecological, biological, and industrial issue.5,6 Dyestuff wastewater treatment methods include adsorption, chemical coagulation,7 liquid membrane separation,8 electrolysis,9 biological treatments,10 oxidation,11 and other processes.12 However, in the treatment of dye wastewaters, the adsorption technique is regarded as a preferred method to remove the pollutants because of its simple design, low cost, easy operation, and reusable features.13,14 Cellulose, most abundant natural polysaccharide on the earth, has numerous alluring properties such as biodegradability, renewability, biocompatibility, high strength and stiffness, and being eco-friendly.15,16 Because of its structural features, cellulose can adsorb dyes directly; however, the adsorption ability of pure cellulose is fairly low. Many © XXXX American Chemical Society

investigators have tried to modify natural cellulose with specific functional groups, such as carboxyl,17 amino,18 mercapto,19 and cyclodextrin20 groups, to improve adsorption ability and remove specific pollutants. Some researchers have recently focused on introducing cationic groups into cellulose to enhance interactions between cellulose and anionic dyes. Pei et al.21 prepared surface quaternized cellulose nanofibrils to have adsorption capacity for the anionic dyes. Liu et al.4 prepared the modification of cellulose with acrylamide and acrylic acid which was demonstrated to effectively increase the adsorption capacity of modified cellulose for anionic and cationic dye. Several reports have also indicated that a particular kind of cellulose-based material can remove dyes with different molecular structures. But there are some disadvantages in these cationic adsorbents. For instance, the amino amount on the cellulose molecules chain is too little to absorb more dyes, and modified cellulose easily dissolves in water, which is not conducive to adsorption. Most of all, many adsorbents do not have the morphology that leads to large adsorption properties. Received: April 27, 2016 Accepted: July 21, 2016

A

DOI: 10.1021/acs.jced.6b00340 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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In the study, in order to get good morphology and further improve the adsorption performance of cellulose to remove anionic dyes, a novel three-dimensional porous CE/PAM-g-QC bead was designed and prepared by the inverse emulsions method. In the adsorbent system, the cellulose acted as a carrier with its strong backbone or skeleton structure, and the PAM-gQC that was prepared by a free radical polymerization method was as an effective adsorbent with abundant aminos. The influences of several parameters (initial concentration, adsorbents dose, contact time, temperature, and pH) were evaluated to determine the best adsorption conditions. Congo red (CR) and Eriochrome blue SE (EBSE) as the model anionic dyes were used to investigate the adsorption behavior of CE/PAM-g-QC beads; the chemical structures and some other properties of Congo red and Eriochrome blue SE are shown in Table 1. CE/PAM-g-QC as a novel adsorbent has a practical and potential effect in the treatment of dye wastewater.

Scheme 1. Synthesis of Polyacrylamide Grafted Quaternized Cellulose (PAM-g-QC)

Table 1. Chemical Structure and Some Properties of Congo Red and Eriochrome Blue SE

washed with ethanol several times. The obtained PAM-g-QC copolymer was dried overnight at 60 °C. For preparation of a series of PAM-g-QC samples with a different grafting ratios, a different quantity of monomer and initiator for each reaction was used; various samples including other details were all listed in Table 2. The final corresponding Table 2. Adsorption Performance of Different Graft Ratios of PAM-g-QC for Removal of CR and EBSE from Aqueous Solutions

2. EXPERIMENTAL SECTION 2.1. Materials. The degreasing cotton was purchased from HeNan Piaoan Group. NaOH, urea, Congo red, and Eriochrome blue SE were purchased from Sinopharm Chemical. (3-Chloro-2-hydroxypropyl) trimethylammonium chloride (CHPTAC), ammonium persulfate (APS), acrylamide (AM), and liquid paraffin Span80 were purchased from Aladdin Reagent. All other reagents were analytical grade and used without further purification before using. 2.2. Preparation of Quaternized Cellulose. The preparation of water-soluble quaternized cellulose (QC) derivatives was similar to that in a previous work.22 Briefly, a certain quantity of CHPTAC solution was added into the 100 g of cellulose solution with constant stirring 400 rpm at 25 °C for 24 h. The final product was neutralized with 0.05 mol L−1 HCl solution, and the resulting product was washed with ethanol. 2.3. Preparation of Polyacrylamide-Grafted Quaternized Cellulose. In an inert atmosphere of nitrogen, the PAM-g-QC copolymer was synthesized by a free radical polymerization method.23 The whole synthetic strategy is shown in Scheme 1. In brief, 2 g of QC was added into 100 mL of distilled water, then it was kept in an oil bath at 70 °C with stirring at 400 rpm until the QC dissolved completely. Then, a certain quantity of acrylamide and ammonium persulfate were added to the reaction system. The reaction continued for 3 h at the same stirring speed and temperature. Then the product was

sample

AM (g)

APS (g)

grafting ratio

CR removal efficiency (%)

CE QC PAM-g-QC1 PAM-g-QC2 PAM-g-QC3 PAM-g-QC4 PAM-g-QC5 PAM-g-QC6 PAM-g-QC7 PAM-g-QC8

null null 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

null null 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0 0 28.74 30.06 70.89 115.15 136.58 153.53 158.61 161.05

14.00 44.35 62.68 60.26 75.30 86.23 90.82 94.72 93.86 94.89

EBSE removal efficiency (%) 14.00 44.35 55.48 53.92 60.81 66.28 69.26 73.97 72.34 73.96

products were named PAM-g-QC1, PAM-g-QC2, PAM-g-QC3, PAM-g-QC4, PAM-g-QC5, PAM-g-QC6, PAM-g-QC7, and PAM-g-QC8, respectively. The grafting ratio was calculated according to eq 1.24 grafting ratio =

100 × N % × MAM Wg × 14

(1)

where MAM is molecular weight of acrylamide, and Wg (g) is the weight of QC. 2.4. Preparation of CE/PAM-g-QC Porous Beads. The CE/PAM-g-QC porous beads were synthesized by an inverse emulsions method;25 the whole strategy is presented in Scheme 2. In the typical procedure, 30 g of solution of CE and PAM-gQC were added into a stirred glass reactor containing 100 mL of liquid paraffin and 3 g of Span80. The reaction was conducted at room temperature for 3 h under mechanical agitation (400 rpm). After that, the final products were washed with distilled water and ethanol several times. The obtained B



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Scheme 2. Schematic Presentation of CE/PAM-g-QC Porous Beads Preparation

CE/PAM-g-QC beads were freeze-dried maintained at −45 °C for 24 h, and the resulting CE/PAM-g-QC beads were shown in Figure 1.

2.6. Experiments of Dyes Adsorption. For the experiments of dyes adsorption, anionic dye CR and EBSE were used in this study. The adsorbent was added into 25 mL of dye solution for 350 min under shaking 150 rpm. The concentration of the two dyes in the aqueous solution was determined by UV−vis spectra (UV-1100D, Shanghai), and the dye concentration was calculated by the absorbance at the maximum absorption; the maximum absorption values of CR and EBSE are listed in Table 1. The adsorption capacity (q) of CR and EBSE on the adsorbent is calculated according to the following equation:26 q=

2.5. Characterization of Materials. The FTIR spectra of CE, QC, PAM-g-QC2, PAM-g-QC4, PAM-g-QC6, and PAM-gQC8 samples in KBr pellets were taken using an AVATAR360 FTIR spectrophotometer (Nicolet, USA) to measure the spectra in the range of 4000−400 cm−1. The Carlo Erba 1108 elemental analyzer was used to measure the elemental content of the samples. Only three elements (carbon, hydrogen, and nitrogen) were estimated, the elemental analysis results are listed in Table 3. The morphology of the adsorbents and their porous structures were characterized by SEM (SU8010, Hitachi, Japan).

dye removal (%) = −1

carbon (%)

hydrogen (%)

nitrogen (%)

43.14 37.37 42.18 41.41 42.76 43.85 44.99 45.86 44.66 44.92

6.20 6.71 7.29 7.20 7.21 7.34 7.33 7.43 7.38 7.45

0.01 1.79 2.83 2.96 7.08 11.34 13.45 15.12 15.62 15.86

Co − Ct 100 Co

(3)

−1

where Co (mg L ) and Ct (mg L ) are the initial and t time concentrations of dye in solution, respectively.

3. RESULTS AND DISCUSSION 3.1. FTIR Characterization. The FTIR spectra of CE, QC, and PAM-g-QC are shown in Figure 2. Compared with that of the pure CE sample, the most relevant difference in the FTIR spectra of QC and PAM-g-QC is the appearance of bands at 1185, 1060, 3200, 1610, and 1410 cm−1. In Figure 2a, the C−H and −OH stretching vibration can be seen from the characteristic peaks at 2983 and 3400 cm−1,28 respectively. According to Figure 2b, the stretching vibration of the C−N group can be seen from the unique characteristic peaks at 1060 and 1185 cm−1, which reveals that quaternary ammonium had been grafted to CE. In Figure 2c−f, the stretching frequency of the −NH2 groups can be seen from the broad absorption band

Table 3. Elemental Analysis Results polymer

(2)

where V (mL) is the volume of solution volume, Co (mg L−1) and Ct (mg L−1) are the dye concentration in the solution at initial and t (min) time, respectively, and m (g) is the adsorbent mass. The effect of initial dye concentration on the adsorption was studied in the range from 100 to 800 mg L−1 with a adsorbent dosage of 1.0 g L−1 at 303 K. The effect of adsorbent dosage on the adsorption was studied in the range from 0.2 to 1.8 g L−1 with a dye initial concentration of 200 mg L−1 at 303 K. The effects of contact time (0 to 350 min), pH (3 to 11), and temperature (303 to 323 K) were also investigated with 1.0 g L−1 of adsorbent. The dye removal was calculated using the eq 3:27

Figure 1. Morphology photos of CE/PAM-g-QC beads.

CE QC PAM-g-QC1 PAM-g-QC2 PAM-g-QC3 PAM-g-QC4 PAM-g-QC5 PAM-g-QC6 PAM-g-QC7 PAM-g-QC8

V (Co − Ct ) m

C



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Figure 2. FTIR of (a) CE; (b) QC; (c) PAM-g-QC2; (d) PAM-g-QC4; (e)PAM-g-QC6; (f) PAM-g-QC8; (g) CE/PAM-g-QC6; (h) CR; (i) EBSE; (j) CR-loaded CE/PAM-g-QC6; (k) EBSE-loaded CE/PAM-g-QC6.

at 3200 and 1610 cm−1 for the copolymers. The absorption peak at 1410 cm−1 could be due to the N−H stretching vibration. With the increase of grafting ratio, the absorption peak strength at 1410 cm−1 increases continuously. This suggests that the polyacrylamide successfully grafted onto the quaternized cellulose. The FTIR spectra of CR and EBSE were shown in Figure 2h,i. The prominent absorption peaks at 1630, 950, and 670 cm−1 were due to the CC, C−H, and CH stretching vibration attached to the benzene ring for CR and EBSE, respectively. Compared with the spectrum of the CE/PAM-gQC6 (Figure 2g), there are new absorption peaks at 955 and 668 cm−1 for CR-loaded CE/PAM-g-QC6 as seen in Figure 2j. These new characteristic peaks were attributed to the C−H stretching frequency and CH bending vibration from the CR molecule. In Figure 2k, the peaks at 950 and 637 cm−1 were assigned to the stretching vibration of C−H and bending vibration of CH for EBSE-loaded CE/PAM-g-QC6. Moreover, the characterization spectra of adsorbent after adsorption showed a new representative peak of CH at 3100 cm−1. These results confirmed that the dyes were successfully adsorbed onto the CE/PAM-g-QC6 molecules. 3.2. Elemental Analysis. Table 3 shows all the results of elemental analysis of CE, QC, and all the graft copolymers. The presence of nitrogen of the CE is extremely little. However, the barely present nitrogen may be due to the trace quantities of unisolated proteins in the polysaccharides. In the QC, the nitrogen content increases to 1.79%, proving that the preparation of quaternized cellulose is successful. In the series of graft PAM copolymers based on QC, a greater percentage of nitrogen may be due to the increased quality of monomer and initiator in the free radical polymerization process. The PAM-gQC6 demonstrates significant nitrogen content from PAM-gQC1 to PAM-g-QC8. Compared with that of the PAM-g-QC6, the change in nitrogen content of PAM-g-QC7 and PAM-gQC8 is not very obvious. 3.3. Structure Characterization. The SEM image (Figure 3) shows the CE/PAM-g-QC6 surface and internal structure of the beads. In Figure 3a, the rough surface of the bead can be seen, showing that the surface has more adsorption sites to remove dye. The internal structure of the bead is presented in Figure 3b where it could be seen that the spherical bead shows the typical well-defined porous and interconnected threedimensional framework structure. It was concluded, during the process of beads forming, that the cross-linking between

Figure 3. SEM of CE/PAM-g-QC6 beads: (a) the surface of the beads, (b) the internal structure of beads, (c) surface before adsorption, (d) surface after adsorption.

cellulose and PAM-g-QC made up the support space structure of the bead, and the evaporation of water during the drying process led to the pore structure.29 Figure 3 panels c and d show contrastive diagrams before and after adsorption. The structure morphology of the bead was examined with a magnification of 20000 and a smooth surface structure without any particles before adsorption was seen(Figure 3c); however, in Figure 3d, many particles were seen on the surface of the bead after adsorption. The result further confirmed that the dyes were tightly adsorbed onto the adsorbents. 3.4. Adsorption Experiments. The data in Table 2 show that with the increase of AM, the dye removal efficiency is better with PAM-g-QC6, so PAM-g-QC6 is chosen to be the best adsorbent. Various factors (dye initial concentration, time, adsorbent dosage, and pH) are investigated to evaluate the adsorption performance of cellulose-based adsorbent. 3.4.1. Effect of Initial Dye Concentration on Adsorption Performance. The adsorption phenomenon was studied with respect to the effect of initial dye concentration. Experiments were conducted with different initial concentrations (Co) of CR D



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Figure 4. Effect of initial dye concentration (a), time (b), adsorbent dosage (c), and solution pH (d) on the adsorption.

dyes was selected as 3.0 to 11.0 in this section. We can see from Figure 4d that the removal efficiency of the CE/PAM-g-QC6 beads continuously decrease with the increase of pH values from 4.0 to 11.0 for CR and 5.0 to 11.0 for EBSE. However, when the pH values were 4.0 for CR and 5.0 for EBSE, the removal efficiency of the adsorbents was the best. As it was known, CR and EBSE are typical anionic dyes and release negative charge under acidic and neutral conditions. At lower pH values (pH < 7), the −NH2 groups in PAM-g-QC6 are protonated to −NH3+ groups. The positive charges of PAM-gQC6 adsorbents were quite strong at pH lower than 7. Therefore, the strong electrostatic attractions generated between cationic adsorbents and anionic dyes. Nevertheless, under alkaline condition, it became more difficult for −NH2 to be protonated and thus the −NH3+ concentration decreased, which is not in favor of electrostatic attractions between adsorbents and negatively charged CR and EBSE dyes, leading to the dye removal efficiency decreased.4 In order to adapt to the adsorption conditions from natural environment, we selected the neutral medium (pH 7.0) for further research. 3.4.4. Effect of Contact Time and Adsorption Kinetics. The effect of contact time was shown in Figure 4b, adsorption equilibrium was reached 100 min for EBSE and 200 min for CR. To obtain a systematic and uniform comparison, 200 min was selected here for the further kinetics investigation of two dyes. To further investigate the adsorption mechanism and its potential rate-controlling steps such as mass transfer, diffusion control, and chemical reaction, the adsorption kinetics models including pseudo-first-order kinetic, pseudo-second-order kinetic, and intraparticle diffusion model were used.32 The pseudo-first-order equations are expressed in linear form as

and EBSE. The removal efficiencies of the two dyes were investigated by changing the initial dye concentration from 100 to 800 mg·L−1 as observed in Figure 4a. With an increase in the initial dye concentration, the removal efficiency of CR and EBSE changed from 99.78% and 99.92% to 46.06% and 41.47%, respectively. The removal efficiencies for the two dyes decreased gradually with the increase of the concentration of the dyes solutions. This decrease may occur because a greater amount of vacant active sites of adsorbent were more available for the adsorption at the lower initial concentration, and then as the concentration increased, the saturated sites made it difficult to capture dye molecules.30 The removal efficiency of CR was higher than the removal efficiency of EBSE at the high initial concentration, which probably was due to the smaller spatial inhibition in the molecular structure of CR and its negative feature making it easier to approach the adsorption sites of cellulose-based adsorbents.15,31 3.4.2. Effect of Adsorbent Dosage on Adsorption Performance. The effect of adsorbent dosage on removal efficiency was investigated by changing various adsorbent amounts from 0.2 to 1.8 g L−1 into the flask containing 25 mL of 200 mg L−1 (CR) and 200 mg L−1 (EBSE) solution at pH = 7.0. The results were presented in Figure 4c. The removal efficiency of CR and EBSE by CE/PAM-g-QC6 adsorbents increased from 65.66% to 98.89% and from 46.24% to 97.53%, respectively. The removal efficiencies of the two dyes increased dramatically with increasing adsorbent dosage up to a concentration of 1.0 g L−1; the removal efficiency reached adsorption equilibrium after 1.0 g L−1, which was due to the increase of available adsorption sites in CE/PAM-g-QC6 adsorbents. The optimum adsorbent dosages for both CR and EBSE were selected as 1.0 g L−1. The removal efficiencies for CR and EBSE reached up to 98.63% and 93.27%, respectively. 3.4.3. Effect of pH Value on Adsorption Performance. In the CE/PAM-g-QC6 spherical beads, PAM-g-QC is prepared by a free radical polymerization method to introduce more amino groups, which were used as an effective adsorbent to remove anion dyes. To investigate the influence of pH on the removal efficiency of CR and EBSE, the pH range for the two

ln(qe − qt ) = ln qe − k1t

(4)

where qt (mg g−1) is the adsorption quantity of CR or EBSE at time t (min); qe (mg g−1) is that adsorption quantity at the equilibrium; k1 (min−1) is the rate constant of the pseudo-firstorder model. E



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Table 4. Kinetic Parameters of Pseudo-First-Order, Pseudo-Second-Order Models and Intraparticle Diffusion for the Adsorption of CR and EBSE onto CE/PAM-g-QC6 Porous Beads pseudo-first-order

pseudo-second-order 2

Co

(qe)exp

(qe)cal

k1

R

100 200 300 400 500 600 700 800

99.781 197.264 284.167 301.286 324.944 332.529 343.179 368.513

58.038 146.072 232.200 261.753 336.938 249.860 217.044 248.788

0.0175 0.0146 0.0145 0.0174 0.0171 0.0155 0.0149 0.0155

0.9611 0.9894 0.9877 0.9821 0.8178 0.9698 0.9913 0.9428

100 200 300 400 500 600 700 800

99.920 186.536 221.921 265.835 277.886 298.872 313.352 331.794

50.552 64.303 137.634 129.205 146.014 141.982 140.457 147.806

0.0175 0.0067 0.0030 0.0050 0.0044 0.0055 0.0058 0.0053

0.9396 0.6586 0.6121 0.6661 0.6707 0.7158 0.7347 0.7339

(qe)cal CR 104.167 208.333 303.030 322.581 344.828 357.143 357.143 384.615 EBSE 105.263 192.308 227.273 277.778 285.714 312.500 322.581 344.828

intraparticle diffusion kp

C

R2

0.9988 0.9979 0.9975 0.9972 0.9964 0.9985 0.9987 0.9970

3.8812 9.4317 14.0945 14.7837 15.7901 15.7695 13.9824 13.0214

40.713 49.400 59.827 69.044 67.151 86.994 124.770 156.300

0.8449 0.8867 0.9049 0.8982 0.9128 0.8606 0.8682 0.8706

0.9980 0.9994 0.9995 0.9992 0.9994 0.9992 0.9993 0.9999

4.3462 7.0534 8.1608 10.8050 11.1211 11.9651 12.1900 11.7798

36.049 86.091 101.552 109.110 115.740 123.641 134.732 157.543

0.7258 0.6268 0.6415 0.6976 0.7041 0.7103 0.6939 0.7012

k2

R

7.122 2.037 1.186 1.182 0.991 1.248 1.704 1.637 6.267 5.878 4.190 3.108 3.086 2.646 2.730 2.739

2

Figure 5. Kinetics of adsorption on the CE/PAM-g-QC6: (a) pseudo-first-order model for CR, (b) pseudo-first-order model for EBSE, (c) pseudosecond-order model for CR, (d) pseudo-second-order model for EBSE.

qt = kp(t 1/2) + C

The pseudo-second-order model is more befitting to understand adsorption on heterogeneous surfaces adsorption and multiple adsorption layers, which is given according to eq 5:33 t t 1 = + qt qe k 2qe2

(6)

where kp (mg g−1 min−1/2) is the rate constant of intraparticle diffusion; C is the constant related to the thickness of the boundary layer, which is in direct ratio to the effect of the boundary layer. The values of kp, C, and R2 are also listed in Table 4. The parameters were graphed from Figure 5 and tabulated in Table 4; it could be seen that the values of R2 for CR and EBSE of pseudo-second-order kinetic model were higher than pseudo-first-order kinetic model in different initial dye concentrations (100 to 800 mg L−1). Moreover, qe for CR and EBSE of the pseudo-second-order kinetic model (Figure 5c,d) was better fitted in the linear plots than that of the

(5)

where k2 (g mg−1 min−1) is the rate constant of pseudo-secondorder model, and t (min), qe (mg g−1), and qt (mg g−1) have the same meanings as those in the pseudo-first-order model. The intraparticle diffusion model34 is also employed to study the kinetic model. The intraparticle diffusion equation is expressed according to eq 6: F



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Figure 6. Plot of qt values versus t1/2 for the adsorption of CR (a) and EBSE (b).

Table 5. Isotherm Data for the Adsorption of CR and EBSE on CE/PAM-g-QC6 Beads Langmuir iostherm

Freundlich isotherm 2

T (K)

qmax

b

R

303 313 323

357.143 370.370 380.084

0.1134 0.1392 0.1338

0.9953 0.9959 0.9971

149.366 177.346 150.912

303 313 323

333.333 344.828 349.284

0.0430 0.0455 0.0557

0.9903 0.9907 0.9920

135.952 141.203 163.155

Kf

Temkin isotherm A

d

R2

0.9236 0.8173 0.7682

0.149 1.121 0.068

77.814 92.051 71.611

0.9708 0.8753 0.8887

0.9841 0.9814 0.9545

0.311 0.348 2.324

99.042 99.716 115.358

0.9172 0.9234 0.9274

n

R

6.5189 7.7042 6.1199 7.4019 7.3964 8.5543

2

CR

EBSE

Where qe is the adsorption capacity at equilibrium (mg g−1), qmax is the maximum adsorption capacity (mg g−1); Ce is the equilibrium concentration of dye (mg L−1); b is the Langmuir constant (L mg−1). The Freundlich isotherm assumes a nonuniform distribution of the heterogeneous surface and a multilayer adsorption onto the surface of adsorbents. The Freundlich equation can be expressed according to eq 8:40

pseudo-first-order kinetic model (Figure 5a,b). The results illustrate that the adsorption of CR and EBSE onto the cellulose-based adsorbents were better fitted to the pseudosecond-order kinetic model. They indicate that the ratecontrolling step in adsorption of the two dyes was the chemisorption between adsorbent and adsorbate.35,36 From Figure 6a,b, we can see that the adsorption of the two dyes by using the porous cellulose-based adsorbents is essentially relevant to three continuous steps. The three stages were circled and indicated in Figure 6. The first stage circled in a blue line indicates external surface adsorption or instantaneous adsorption process. The second stage circled in red line is the gradual adsorption process, for which the intraparticle diffusion was rate-controlled. The third stage circled in the green line is the final equilibrium adsorption process, in which the intraparticle diffusion rate slowed down owing to the extremely low concentration of the dyes in solution.34 3.4.5. Adsorption Isotherms. To study the relativity between adsorption capacity and equilibrium concentration, adsorption isotherm models are used. The isotherm parameters for the adsorption of CR and EBSE were analyzed in the Langmuir isotherm, Freundlich isotherm, and Temkin isotherm.37 The adsorption of CR and EBSE with initial concentrations of two dyes ranging from 100 to 800 mg·L−1 was studied at pH = 7.0, with an adsorption time t = 200 min in different temperature T (303, 313, and 323 K). The Langmuir isotherm supposes monolayer adsorption of adsorbate onto the adsorbent surface, and the equal adsorption activation energy is distributed to each molecule on the surface. The Langmuir equation can be expressed according to the following eq 7:38,39 Ce C 1 = + e qe bqmax qmax

ln qe = ln K f + ln

Ce n

(8)

−1

where qe (mg g ) is the equilibrium adsorption capacity according to Freundlich isotherm; Ce (mg L−1) is the equilibrium concentration of dye; Kf (L mg−1) and n are Freundlich constants connected with the adsorption capacity and adsorption intensity, respectively. The phenomenon of adsorption of heterogeneous systems is described by the Temkin isotherm. The Temkin isotherm supposes that the adsorption heat decreases linearly with increasing interactions between adsorbents and adsorbates, and the adsorption binding energies are distributed uniformly. The Temkin model27 can be expressed as ln qe =

RT ln(ACe) d

(9)

and it can be linearized according to eq 10: qe = B ln A + B ln Ce

(10)

where B = RT/d, Ce and qe are the same meaning as mentioned previously, and A (L mg−1) and d (J mol−1) are the Temkin constants. R (8.314 J mol−1 K−1) is the universal gas constant, and T (K) is the absolute temperature (303, 313, and 323 K). According to the information in Table 5, the relative parameter values are calculated from the Langmuir, Freundlich, and Temkin isotherm models. The Langmuir model shows that

(7) G



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Figure 7. Effect of temperature in different initial CR (a) and EBSE (b) dyes concentration.

the maximum theoretical adsorption capacity is 380.084 mg g−1 for CR and 349.284 mg g−1 for EBSE, which are similar to the experimentally obtained values at 323 K of 386.648 mg g−1 for CR and 345.459 mg g−1 for EBSE. The values of b are found to be within the range from 0 to 1, indicating that the adsorbents are favorable for adsorption of the two dyes. From the Freundlich isotherm model, the values of Kf explain that the adsorption of CR and EBSE on the adsorbents is easy and adsorption capacity increases with increasing initial dye concentration. The scopes of n explain the favorability and capacity of the adsorption system. Calculated from the Freundlich model, n > 1 confirms favorable adsorption conditions. From the Temkin isotherm model, the higher value of d demonstrates a strong interaction force between adsorbents and absorbates. An analysis of the results obtained from the three isotherm models shows that the Langmuir model has higher R2 (>0.99) values than the R2 of other two isotherm models, which indicates that the adsorption of the two dyes on the CE/PAM-g-QC6 adsorbents is a monolayer adsorption.4 3.4.6. Effect of Temperature and Thermodynamic Studies. Figure 7 shows the effect of temperature on the adsorption capacity of CR and EBSE dyes by CE/PAM-g-QC6 adsorbent. The experiments were carried out at different temperatures (303, 313, and 323 K). As observed in Figure 7 panels a and b, the adsorption capacities for the two dyes increased with the increase of the initial concentration of dyes solutions, and the adsorption capacity became higher as the temperature increased for the different dye initial concentrations. It can be easily understood that the adsorption of CR and EBSE on the adsorbent is an endothermic process implying a chemical adsorption process.41 To study the fluctuation information on the energetic changes and effect of temperature on adsorption process of the two dyes, the thermodynamic parameters including the Gibbs free energy change (ΔGo), enthalpy change (ΔHo) and entropy change (ΔSo)42 were investigated. These parameters were calculated according to the following eq 12 and eq 13:2,33,43 Kc =

CA Ce

the absolute temperature, and Kc (L mol−1) is the equilibrium constant. To confirm the accuracy of our experiment, thermodynamic studies were completed in different initial dye concentrations (300, 500, and 700 mg L−1). According to the information in Table 6, the negative values of ΔGo for CR and EBSE indicate Table 6. Thermodynamic Parameters at Different Temperatures Co (mg L−1) 300

500

700

300

500

700

(12)

ΔGo = −RT ln Kc

(13)

303 313 323 303 313 323 303 313 323 303 313 323 303 313 323 303 313 323

CR −2.710 −2.800 −2.889 −8.495 −8.776 −9.056 −11.183 −11.553 −11.922 EBSE −3.483 −3.599 −3.714 −7.784 −8.041 −8.298 −9.629 −9.947 −10.265

ΔHo (kJ mol−1)

ΔSo (J mol K−1)

1.947

8.951

6.952

28.059

9.209

36.939

4.083

11.510

7.230

25.712

8.338

31.806

their spontaneous nature of adsorption process. The negative value of ΔGo increases at higher temperatures in different initial dye concentrations which confirms that the adsorption quantity at equilibrium must increase with increasing temperature. The positive value of ΔHo in the temperature range from 303 to 323 K again shows the endothermic and chemisorption nature of the adsorption process.44 The higher temperature was favorable to adsorption, and CR and EBSE molecules are adsorbed orderly on the surface of cellulose-based adsorbents. Eventually, the positive value of ΔSo proves a good affinity of CR and EBSE for the adsorbents and also confirms that the adsorption process randomly takes place at the solid solution interface.45,46 3.5. Adsorption and Desorption Mechanism of CR and EBSE. According to the analysis of the adsorption kinetics, isotherm, and thermodynamics, it can be considered that the adsorption of CR and EBSE was due to the chemical action

(11)

⎛ ΔS o ΔH o ⎞ ⎟ − ln Kc⎜ ⎝ R RT ⎠

T (K) ΔGo (kJ mol−1)

where R (8.314 J mol−1 K−1) is the ideal gas constant, CA (mg L−1) is the amount adsorbed on solid at equilibrium, T (K) is H



Article

Scheme 3. Schematic Drawing for the Possible Interactions between the Adsorbent and CR and EBSE Dye Molecules

between anionic dyes and cellulose-based adsorbents. In general, the change of solution pH was related to the method of interaction (electrostatic attraction or repulsion) between adsorbents and adsorbates. Especially, the effect studies of pH demonstrated that the electrostatic interactions between dyes and the adsorbents were the main interaction forces, because the removal efficiency of the adsorbent had a higher value under neutral or mildly acidic conditions as indicated in Figure 4d. The −SO3− groups on the anionic dyes and −NH2 groups at the adsorbents generated electrostatic interactions. Scheme 3 shows the suggested adsorption and desorption mechanism between the adsorbent and dyes. To further illustrate the mechanism, desorption studies were applied to help elucidate the mechanism of the adsorption process. The regeneration study of adsorbent is key to economy and reutilization. Under strong alkaline conditions, the anionic dye could be desorbed from the cationic adsorbent, which proved that the electrostatic adsorption was the main interaction force. Table 7 shows the effect of desorption

Table 8. Comparison with Other Cellulose-Based Adsorbents adsorbent cellulose matrix CaCO3−cellulose aeroge cellulose-based adsorbent cellulose nanofibrils amino-functionalized nanocrystalline cellulose polyacrylamide grafted quaternized cellulose polyacrylamide grafted quaternized cellulose

CR desorption efficiency (%)

EBSE desorption efficiency (%)

pH 9.0 in water pH 10.0 in water pH 11.0 in water DMF DMSO

6.32 9.25 12.42 25.65 36.62

8.76 11.30 16.49 28.85 40.83

qmax (mg g−1)

ref

Reactive red RB Congo red Reactive blue 21 Reactive orange 16 Acid red GR

8.94 75.81 200.00 295.10

47 48 49 50

555.60

38

Congo red

380.08

Eriochrome blue SE

349.28

this study this study

4. CONCLUSIONS The CE/PAM-g-QC adsorbents were designed and synthesized to remove the anionic dyes. The modification of cellulose with CHPTAC and AM could effectively increase its amino content, leading to an increase in adsorption capacity. The adsorbents showed a porous and three-dimensional framework structure by SEM. The experimental results of CE/PAM-g-QC fit well to the pseudo-second-order kinetic model and Langmuir isotherm, and the maximum theoretical adsorption capacity of the beads was 380.084 mg g−1 for CR and 349.284 mg g−1 for EBSE. The thermodynamic studies provided evidence for the endothermic and spontaneous nature of the adsorption process. The new adsorbents were remarkable in adsorption capacity according to a comparison with other cellulose-based adsorbents. Above all, CE/PAM-g-QC beads could be regarded as a kind of cost-efficient adsorbent in future water treatment work.

Table 7. Desorption of CR and EBSE from Cellulose-Based Adsorbents solvent

anionic dye

efficiency in different solvents. When the pH increases from 9.0 to 11.0 in water, the desorption efficiency increases from 6.32% to 12.42% for CR and from 8.76% to 12.49% for EBSE. It illustrates that the change of pH can affect the desorption efficiency, which further confirms that the electrostatic interaction is the main force between the dyes and the adsorbents. Although these dyes can be desorbed, the desorption efficiency is low not only in alkaline conditions but also in organic solvent; the maximum desorption efficiency is 36.62% for CR and 40.83% for EBSE in DMSO solvent. So further studies are being performed at different experimental conditions (for example, different temperature, different solvent types, and so on). 3.6. Comparison with Other Cellulose-Based Adsorbents. We demonstrate the anionic dyes adsorption capability of CE/PAM-g-QC adsorbent by comparing with other cellulosebased adsorbents as listed in Table 8. It can be seen that the anionic dyes adsorption capability of CE/PAM-g-QC adsorbent is greater than that reported for most other cellulose-based adsorbents, which confirms that the new CE/PAM-g-QC adsorbent has potential and practical value for anionic dye.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 0086-0993-2057276 or +86-13677533280. E-mail: [email protected]. Funding

This work was supported financially by funding from the National Natural Science Foundation of China (21367022 and 21467024) and Bingtuan Innovation Team in Key Areas (2015BD003). Notes

The authors declare no competing financial interest.

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Removal of Anionic Dyes from Aqueous Solutions by Cellulose-Based

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