Removal of Crystal Violet by a Novel Cellulose-Based Adsorbent

Article pubs.acs.org/IECR

Removal of Crystal Violet by a Novel Cellulose-Based Adsorbent: Comparison with Native Cellulose Yanmei Zhou,* Min Zhang, Xinhai Wang, Qi Huang, Yinghao Min, Tongsen Ma, and Jingyang Niu Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P.R. China ABSTRACT: A novel cellulose-based adsorbent (CGS) was synthesized by a facile, two-step modification route. It was characterized by the FTIR, XRD, and SEM technique. The adsorption characteristics of CGS were compared with those of native cellulose using crystal violet (CV) as an adsorbate in this work. The effects of contact time, temperature, pH, and initial concentration on the sorption were all investigated. The adsorption kinetics for both CGS and cellulose were found to follow a pseudo-second-order (P-S-O) kinetic model. The equilibrium data of CGS fitted well with the Langmuir isothermal model, and the maximum theoretical adsorption capacity of CGS for CV was 218.82 mg g−1 at 50 °C; the adsorption increased by 70.8% compared to that of native cellulose. A thermodynamics study was also performed. The comparative study confirmed that the structure of CGS is more conducive for the adsorption of CV than cellulose. Furthermore, adsorption/desorption experiments showed that CGS could be reused for at least eight cycles with stable sorption ability after being regenerated with 50% ethanol solution (pH 3.0).

1. INTRODUCTION Dyes are widely used in the textile, leather, paper, plastics, printing, electroplating, food, and cosmetics industries.1 Vast amounts of synthetic dyes together with industrial effluents are released into the aquatic environment, and most of them are difficult to decontaminate, because they have complex organic structures.2 Even when the amount of dyes present in water is very small (less than 1 ppm for some dyes), the presence of the dyes is highly visible and undesirable.3 For example, crystal violet (CV) is one common type of dye in wastewater, widely used in the textile industry. Generally, these organic compounds are also teratogenetic, carcinogenic, and mutagenic and pose serious threats to human health worldwide, as well as to marine life through biological enrichment in the material cycle.4,5 Therefore, dyes must be efficiently removed from wastewater prior to its reuse or discharge into the aquatic environment. Up to now, the main techniques for the removal of dyes from wastewater have been solvent extraction,6 chemical precipitation,7 photocatalysis,8 membrane filtration,9 ion exchange,10 electrochemical treatment,11 adsorption,12 and bioremoval.13 However, the adsorption technique is considered as a superior technique for the treatment of dyestuff wastwaters because of its simplicity in design, low initial cost, ease of operation, reusability, and insensitivity to toxic pollutants.14 Cellulose, an abundant biopolymer in nature, is biodegradable and environmentally friendly. It can be used as an inexpensive adsorbent, adsorbing dyes directly owing to its structural features.15 To improve adsorption ability and efficiency, many investigators have attempted to modify natural biopolymers with specific functional groups, such as carboxyl,15,16 amino,17,18 mercapto,19 cyclodextrin,20 oximido,4 and olefin21 groups, to remove specific pollutants, through physical, chemical, or other more complicated interactions. In this study, we used glycidyl methacrylate and sulfosalicylic acid to modify cellulose on the basis of literature research, © 2014 American Chemical Society

obtained the new adsorbent CGS (i.e., cellulose modified with glycidyl methacrylate and sulfosalicylic acid), and performed a comparative study with native cellulose. Characterization analysis was done before and after adsorption using Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The results indicated that the adsorption of CV on CGS was significantly improved compared to that on native cellulose. The theory of thermodynamics and adsorption isotherm both changed, which implies that fresh force militated in the adsorption process.19,22,23 Furthermore, adsorption/ desorption tests attested to its good cycle performance. We also discuss the adsorption mechanisms of the adsorption of CV onto CGS. Thus, this work might open a new possibility for the achievement of desirable adsorbents.

2. EXPERIMENTAL SECTION 2.1. Reagents and Instruments. Cellulose was provided by Henan Xiren Cellulose Co., Ltd. (Henan, China). Glycidyl methacrylate (>99%, GMA) was obtained from Shangqiu Shengyuan Industrial Assistant Co., Ltd. (Henan, China). Crystal violet (>99%, CV), sulfosalicylic acid (SSA), and sodium hydroxide were all provided by Enterprise Group of Chemical Reagent Co., Ltd. (Henan, China). All aqueous solutions and standards were prepared using distilled water. The following instruments were employed in this work: pH meter (PHs-29 Type, Shanghai Precision & Scientific Instrument Co., Ltd., Shanghai, China), ultraviolet spectrophotometer (TU-1900, Beijing Purkinje General Instrument. Co., Ltd., Beijing, China), and shaker (HZQ-F160A Shanghai A Constant Scientific Instrument Co., Ltd., Shanghai, China). Received: Revised: Accepted: Published: 5498

December 6, 2013 March 1, 2014 March 1, 2014 March 1, 2014 dx.doi.org/10.1021/ie404135y | Ind. Eng. Chem. Res. 2014, 53, 5498−5506

Industrial & Engineering Chemistry Research

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Figure 1. Synthesis and structure of CGS.

2.2. Synthesis of CGS. The synthesis procedure of CGS (i.e., cellulose modified with GMA and SSA) is presented in Figure 1. Native cellulose was first basified and then modified with GMA to obtain epoxy cellulose according to the method described in our previous work.24 Next, 1.0 g of the prepared epoxy cellulose was dissolved in 50 mL of distilled water with stirring for 15 min at room temperature under a nitrogen gas atmosphere, after which 1.3 g of SSA and 0.3 g of NaOH were added with constant stirring for 6 h at 85 °C. Finally, the obtained adsorbent CGS was separated through leaching, repeatedly washed with hot distilled water, and finally dried at 60 °C in a vacuum oven until complete dryness. 2.3. Characterization of CGS. The intermediate product and the final adsorbent were measured on FTIR spectroscopy (Thermo Nicolet Avatar 360 FTIR spectrometer) in transmittance mode using KBr pellets. Measurements were taken in the wavelength range from 400 to 4000 cm−1. Scanning electron microscopy (SEM) analysis was performed using a JEOL JSM5600LV scanning electron microscope. X-ray diffraction (XRD) patterns were recorded on an X’Pert Pro X-ray diffractometer with Cu Kα radiation at a wavelength of λ = 0.1541 nm, and the generator worked at 40 kV and 40 mA. Diffractograms were collected in the 2θ range of 10−40° at a rate of 0.05°/s. 2.4. Adsorption Experiments. For each experiment, 25 mg of adsorbent was placed in a 100 mL conical flask and then mixed with 25 mL of CV solution. To measure the sorption kinetics, CV solutions with three different initial concentrations (i.e., 50, 100, and 200 mg L−1) were used, and the adsorption was measured at time intervals ranging from 5 to 150 min. To obtain sorption isotherms, CGS in CV solutions of different initial concentrations (from 50 to 450 mg L−1) was shaken for 150 min at different temperatures. The same experiments were performed with cellulose. The pH of the CV solution, measured with a pH meter, was adjusted using solutions of sodium hydroxide (0.01 mol L−1) and muriatic acid (0.01 mol L−1). After the mixture had been shaken in controlled shaker for a scheduled time at 190 rpm at different temperatures, the adsorbent was filtered. The CV concentration of the filtrate was determined by ultraviolet spectrophotometer. The equilibrium adsorption capacity of CV, qe (mg g−1), was calculated according to the equation

qe =

(C0 − Ce)V W

(1)

where C0 (mg L−1) is the initial concentration of CV solution, Ce (mg L−1) is the equilibrium concentration of the CV solution after adsorption, V (L) is the volume of CV solution, and W (g) is the quantity of adsorbent used. 2.5. Desorption and Reusability Behaviors of CGS. We performed dynamic desorption experiments to investigate the recyclability of CGS. For these experiments, 50 mg of CGS was packed into a glass column, and then 25 mL of CV solution (200 mg L−1) was passed through the column at a constant flow rate of 0.5 mL min−1. After the column had been washed with distilled water, 25 mL of 50% ethanol solution (pH 3) was added for desorption at the same rate as used for the CV solution (0.5 mL min−1). This cycle was repeated until the adsorption capacity decreased rapidly.

3. RESULTS AND DISCUSSION 3.1. FTIR Spectroscopy Characterization. The FTIR spectra of CGS, epoxy cellulose, and CGS−CV complex (CGSCV) are shown in Figure 2. Comparison of the spectra of epoxy cellulose and CGS indicates that, in the latter, the

Figure 2. FTIR spectra of epoxy cellulose, CGS, and CGSCV. 5499

dx.doi.org/10.1021/ie404135y | Ind. Eng. Chem. Res. 2014, 53, 5498−5506


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41.31%) appeared to decline greatly compared to that of native cellulose (χc = 77.22%),24 which indicates that the wellorganized structure of cellulose changed. This change was with respect to the diversified interactions (chemical and physical) between each group. As the reactions occurred on the cellulose surface, the intensity of the interactions decreased, causing a nonuniform distribution.9,27 3.3. SEM Characterization. SEM images of native cellulose, CGS, and CGSCV are shown in Figure 4. The

representative peaks of allyl glycidyl ether at 3075 and 3010 cm−1 and the characteristic peaks of the epoxy group at 750, 847, and 905 cm−1 all disappeared. In the CGS spectrum, the band at 1162 cm−1 is due to antisymmetric stretching vibrations of the COC bridge,21,25,26 the low-intensity peak at 1383 cm−1 represents hydroxyl groups attached to the benzene ring,27 and the presence of SO3 groups in CGS is confirmed by the band at 1275 cm−1.21,28 The presence of COOH groups in CGS is confirmed by the characteristic CO stretching and COH in-plane bending peaks at 1275 and 1383 cm−1, respectively. Moreover, we performed FTIR characterization of CGSCV after washing it a few times and observed the representative peak of NH at 1591 cm−1, confirming that CV was adsorbed on CGS.15 Together, these results confirmed the presence of GMA and SSA in CGS and the formation of a CGS−CV complex, which achieved the purpose of removing CV from solution. 3.2. X-ray Diffraction Characterization. Figure 3 presents the X-ray crystal diffraction patterns of cellulose and

Figure 3. X-ray diffraction patterns of CGS and cellulose.

CGS in the range between 10° and 40°. The typical diffraction peak of native cellulose is still clear in the pattern of CGS, which indicates that the linear structure of glucose rings remained, as the modifications of cellulose were expected to occur first on the available hydroxyl groups located on the cellulose surface. However, in the CGS diffraction pattern, another typical diffraction peak of native cellulose at 15.1° shifted to 14.7°, and fresh diffraction peaks at 2θ = 26.27° and 34.2° appeared that identified the presence of SO3H.27,29 Because the positions of the diffraction peaks of COOH and SO3 were similar, the two peaks merged into one.15,30 Simultaneously, the changes in the crystal structures were investigated in terms of crystallinity. The crystallinity (χc, %) was estimated by Rabek’s method31 as χc =

Sc Sc + Sa

(2)

where Sc is the sum of the areas of the crystal diffraction peaks of the sample and Sa is the sum of the areas of the amorphous diffraction peaks of the sample. The crystallinity of CGS (χc =

Figure 4. Morphology contrast among cellulose, CGS, and CGSCV (magnification: cellulose, 1000×; CGS, 1000×; CGSCV, 1000×). 5500



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a plateau as a result of the saturation of active sites.33 We chose pH 9.0 in adsorption experiments for CGS and native cellulose to obtain a better contrast. The adsorption of CV by CGS was higher than that by native cellulose, because of the strong electrostatic interactions between CV and the functional groups of CGS. 3.5. Effects of Contact Time on Adsorption and Adsorption Kinetics. The CV uptake as a function of time was investigated at different initial concentrations. As shown in Figure 6 a, for cellulose, the CV uptake reached adsorption equilibrium rapidly, within 20 min. However, adsorption equilibrium was reached nearly within 150 min for CGS. To obtain a better comparison, we chose the same shaking time of 150 min in this study. The pseudo-first-order (P-F-O) and pseudo-second-order (P-S-O) equations are expressed in linear form as34,35

SEM images of native cellulose appear smooth and full of threads. Cellulose microfibrils are typically composed of 1000− 2000 glucan chains of the same polarity and are spatially separated from each other.30 Through the modification process, the cellulose bundles were split and degraded, as shown in the SEM image of CGS. The surface became rough, with many fractures and small convex protrusions. The SEM image of CGSCV also appeared rough, with severe swelling. 3.4. Effects of pH on Adsorption. As shown in Figure 5, pH is an important parameter for both CGS and cellulose, as

ln(qe − qt ) = ln qe − k1t

(3)

t 1 1 = + t qt qe k 2qe 2

(4) −1

−1

−1

respectively, where k1 (min ) and k2 (g mg min ) are the corresponding rate constants, qe (mg g−1) is the adsorption capacity at adsorption equilibrium, and qt (mg g−1) is the adsorption capacity at adsorption time t (min). qe and k1 can be calculated from the slope and intercept of a plot of eq 3, whereas qe and k2 can be calculated from the slope and intercept of a plot of eq 4. In eq 4, when t → 0, the adsorption rate is denoted as h and is calculated as Figure 5. Variation of adsorption with pH. Conditions for CGS and cellulose: 20 °C, initial concentration of 50 mg L−1, shaking for 150 min.

h = k 2qe 2

(5) 34,35

The Elovich equation is expressed in linear form ⎛1⎞ ⎛1⎞ qt = ⎜ ⎟ ln(αβ) + ⎜ ⎟ ln t ⎝β⎠ ⎝β⎠

adsorption increased rapidly as pH rose from 2.5 to 10. In contrast, for pH values above 11 or below 2.3, CV would transmute. Cationic dyes are favorably adsorbed on negatively charged surfaces.32 In strongly acidic solutions, both CGS and cellulose adsorbents were protonated. Because H+ was present in higher concentration and had a smaller ionic radius, it was adsorbed more easily than crystal violet,3 and the repulsive forces between the protonated adsorbent and CV molecules dominated the adsorption process for CGS and cellulose, leading to low adsorption. When the pH was increased to 8 and 9, the protonation decreased slowly, and the electrostatic interaction between the adsorbents and CV enhanced. As shown in Figure 5, adsorption first increased and then reached

as

(6) −1

−2

where α is the initial adsorption rate (mg g min ) and β is the desorption constant (g mg−1 min−1), which is related to the activation energy and the extent of surface coverage for chemisorption.36 According to the Elovich equation, a plot of qt versus ln t gives the 1/β and (1/β)ln(αβ) as the slope and the intercept, respectively. The pseudo-first-order (P-F-O), pseudo-second-order (P-SO), and Elovich equations are used to describe adsorption kinetics on solid surfaces and were used in this work because adsorption kinetics can provide valuable information regarding the mechanism of the adsorption process in the treatment of

Figure 6. Variation with time of the adsorption of CV: (a) cellulose and (b) CGS. Conditions: 20 °C, pH 9.0. 5501



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Table 1. Kinetic Parameters of Three Models (P-F-O, P-S-O, and Elovich) Pseudo-First-Order Model CGS −1

initial concentration (mg/L)

qe (mg g )

50 100 200

7.70 54.14 100.28

cellulose −1

k1 (min )

R

2

0.011 0.8899 0.005 0.8033 0.011 0.9059 Pseudo-Second-Order Model

−1

qe (mg g )

k1 (min−1)

R2

13.14 10.01 2.06

0.0013 0.0018 0.0013

0.47138 0.3787 0.31274

CGS initial concentration (mg/L) 50 100 200

k2 (g mg

−1

−1

min )

0.0042 0.0004 0.0002

h (mg g

cellulose −1

−1

min )

2

R

10.7411 0.9998 3.8373 0.9718 4.2425 0.9699 Elovich Model

k2 (g mg

−1

−1

min )

0.0336 0.0310 0.0184

CGS initial concentration (mg/L) 50 100 200

α (g g

−1

−1

min )

168375.1 46.04 1.05

h (mg g−1 min−1)

R2

4.7239 6.6855 28.6629

0.9927 0.9830 0.9998

cellulose −1

β (g mg )

R

0.3263 0.0761 0.1563

2

0.9544 0.7540 0.8413

−1

α (g g

−1

min )

1.75 × 10 5.73 × 109 1.28 × 1022 13

β (g mg−1)

R2

1.2653 0.6158 5.8439

0.33843 0.35423 0.30033

Figure 7. Variation in the adsorption of CV with initial concentration: (a) cellulose and (b) CGS. Conditions: pH 9.0, shaking for 150 min.

effluents.35,36 The parameters obtained by fitting plots of these equations to the experimental data are presented in Table 1. According to all of the information in Table 1, the adsorption of CV onto CGS and cellulose can be best explained with the P-S-O kinetics model, as its R2 value was highest. This suggests that the adsorption process involves valence forces through the exchange or sharing of electrons between adsorbent and adsorbate,22,37 which conforms with the original molecular design concept of this work, increasing the active sites (SO3, COOH) on the CGS sorbent surface.38,39 Although dye particles accumulated on the active sites of the adsorbents as the adsorption time increased, forming a plateau,40 the repulsive forces between the dye molecules on the adsorbent and in the bulk phase played the major role.34 Additionally, the values of α and h suggest that the adsorption process was fast. The value of k2 decreased as the initial concentration of dye solution increased, because of the increase in adsorption as the initial concentration increased.41 3.6. Isothermal and Thermodynamic Studies of Adsorption. As shown in Figure 7, the effects of temperature and initial concentration were investigated. The uptake of dye was found to be minimally affected by temperature, so we chose 20 °C in this study for convenience. To obtain the adsorption energy and entropy, to investigate whether the adsorption process would take place spontaneously, thermodynamic

parameters including the Gibbs free energy change (ΔG0), enthalpy change (ΔH0), and entropy change (ΔS0) were calculated using the equations q Kc = e M Ce (7) ln(Kc) =

ΔS 0 ΔH 0 − R RT

(8)

ΔG 0 = −RT ln Kc

(9) −1

−1

where R (8.314 J mol K ) is the ideal gas constant, M (407.98 g mol−1) is the molar mass of CV, T (K) is the absolute temperature, and Kc (L mol−1) is the equilibrium constant of adsorption. The values of ΔH0 and ΔS0 were determined from the slopes (−ΔH0/R) and intercepts (ΔS0/R) of plots of ln(qe/ Ce) versus 1/T. Furthermore, the values of these parameters indicate the type of adsorption force according to the following general guidelines:23,35 4 < ΔH0 < 10 kJ mol−1, van der Waals forces; 2 < ΔH0 < 40 kJ mol−1, hydrogen-bonding forces; 2 < ΔH0 < 29 kJ mol−1, dipole−dipole interactions; ΔH0 > 60 kJ mol−1, chemical bonding forces; ΔH0 ≈ 40 kJ mol−1, exchange of dentate; −20 < ΔG0 < 0 kJ mol−1, physical adsorption; and −400 < ΔG0 < −80, chemical adsorption; 5502



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adsorbent surface at a constant temperature. The Langmuir isotherm equation is given by13,45

The related parameters are reported in Table 2. For CGS, when the initial concentration was 50 mg L−1, at the

Ce C 1 = + e qe qmbL qm

Table 2. Results for Related Parameters of Adsorption Thermodynamics initial concentration (mg/L)

temperature (°C)

ΔG0 (kJ mol−1)

ΔH0 (kJ mol−1)

ΔS0 (J mol−1 K−1)

10 20 30 40 50

−5.24 −0.76 −10.56 −7.08 −10.41

43.12

164.69

50

(10)

where qe (mg g−1) and Ce (mg L−1) are the equilibrium adsorption capacity and equilibrium concentration of dye in solution, respectively; qm (mg g−1) is the maximum adsorption capacity; and bL (L mg−1) is a parameter related to the adsorption energy. qm and bL are calculated from the slope and intercept of a linear plot of the Langmuir isotherm equation. The Freundlich isotherm equation is given by13,45 ln qe = ln KF +

investigated temperatures, the ΔG0 value ranged from −10.56 to −0.76 kJ mol−1, which is in the scope of −20 < ΔG0 < 0 kJ mol−1, meaning that the process was spontaneous physical adsorption. ΔH0 (43.12 kJ mol−1) at the investigated temperatures reflects that the process is the result of dentate and physical interactions, such as van der Waals, hydrogenbonding, and electrostatic forces. The positive value of ΔH0 also indicates that the adsorption process is decalescent; the positive ΔS0 value confirms that randomness increases at the solid−solution interface during the adsorption process.21,42 Figure 7 clearly reveals that the equilibrium adsorption on CGS was much higher than that on native cellulose, which further indicates that the COOH and SO3H groups on CGS enhanced the adsorption capacity greatly.43,44 To study the dye adsorption behavior on CGS further, the obtained adsorption data were analyzed based on the Langmuir, Freundlich, and Temkin isotherm equations, as these equations reflect the specific relationship between the adsorbate and

1 ln Ce nF

(11)

where KF and nF represent Freundlich constants measuring the adsorption capacity and the adsorption intensity, respectively. Finally, the Temkin isotherm equation is given by13,45 qe =

RT RT ln A T + ln Ce bT bT

(12)

where AT (mL mg−1) and bT (J mol−1) are the isotherm constant and Temkin−Pyzhev constant, respectively. The simulation results based on the aforementioned models are listed in Table 3. This table shows that the Langmuir isotherm best fit the experimental data for CGS at different temperatures. The maximum adsorbance of CV at 50 °C calculated using this model was 218.82 mg g−1, which is close to the results of actual tests that the maximum adsorption capacity of CV was found to be 189.78 mg g−1 for an initial solution concentration 450 mg L−1 at 50 °C.

Table 3. Parameters of the Adsorption Isotherm Models Langmuir Isotherm Model CGS temperature (°C)

Qm (mg/g)

bL (L/mg)

10 20 30 40 50

171.23 175.43 169.92 201.21 218.82

0.03 0.04 0.11 0.02 0.02

cellulose R2

RL

0.42−0.07 0.9720 0.44−0.08 0.9987 0.13−0.02 0.9969 0.54−0.12 0.9021 0.53−0.11 0.8970 Freundlich Isotherm Model CGS

temperature (°C)

KF (L/g)

nF

10 20 30 40 50

25.59 23.96 55.21 29.22 41.67

3.19 2.80 4.94 3.30 4.03

Qm (mg/g)

bL (L/mg)

RL

R2

198.02 304.88 209.64 165.84 118.20

0.0036 0.0021 0.0033 0.0052 0.0077

0.85−0.38 0.90−0.51 0.86−0.40 0.79−0.30 0.72−0.22

0.7749 0.5863 0.7517 0.7415 0.7601

cellulose R2 0.9688 0.9740 0.9919 0.9021 0.8966 Temkin Isotherm Model

KF (L/g)

nF

R2

2.69 2.15 2.84 4.50 5.91

1.57 1.43 1.59 1.82 2.14

0.9415 0.9368 0.9370 0.8586 0.8175

CGS

cellulose 2

temperature (°C)

AT (L/mg)

bT (J/mol)

R

10 20 30 40 50

0.66 0.53 2.63 0.85 2.92

84.09 72.95 125.79 89.44 107.45

0.9301 0.9802 0.9925 0.8636 0.8252 5503

AT (L/mg)

bT (J/mol)

R2

0.06 0.05 0.06 0.09 0.12

65.54 57.85 70.92 85.73 106.27

0.9072 0.91523 0.93086 0.85007 0.78746



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Additionally, a dimensionless constant, RL, can be calculated based on the Langmuir constant bL that reflects the essential characteristic of the Langmuir model. RL is defined as RL =

1 1 + bL C 0

(13)

The parameter RL indicates the type of isotherm according to the following general ranges: RL > 1, unfavorable; RL = 1, linear; 0 < RL < 1, favorable; and RL = 0, irreversible. Furthermore, the smaller the RL value, the greater the affinity between the adsorbent and the adsorbate.39,46 The values of RL ranged from 0.02 to 0.54 for different initial concentrations of dye solution at different temperatures, indicating that the adsorption of CV on CGS was favorable. In contrast, for native cellulose, the Freundlich isotherm was found to explain the experimental data best at different temperatures. The values of nF ranged from 1.56 to 2.14 at different temperatures, which is within the range of 1−10,36 revealing that the adsorption was again favorable. We now focus on the value of AT in the Temkin adsorption isotherm. For CGS, it ranged from 0.53 to 2.92, and for native cellulose, it ranged from 0.05 to 0.12 under different conditions, which indicates that the potential of CGS for adsorption of CV is higher than that of cellulose. The reason for the difference is most likely the electrostatic attraction between COOH and SO3H groups and dye molecules.30,47 Moreover, the Temkin constant bT ranged from 72.95 to 125.79 J mol−1 for CGS and from 65.54 to 106.27 J mol−1 for CGS for native cellulose, which indicates that the interactions between the sorbate and the sorbents were weak (physisorption), because the typical range of bonding energies for an ion-exchange mechanism (chemisorption) is 8000−16000 J mol−1.35,44 On the basis of all of these results, we can propose an adsorption mechanism. The adsorption of CV onto both CGS and native cellulose can be fit to the P-S-O model, which states that valence forces through the exchange or sharing of electrons between the adsorbent and adsorbate play a key role during adsorption. The adsorption rate usually depends on the number of available adsorption sites on the surface of the adsorbent.13,48 The surface modification of cellulose through the grafting of SSA and GMA for the formation of CGS sorbent was found to influence and alter the CV sorption characteristics, as a result of an increase in the available adsorption sites. For CGS, the adsorption process involves monolayer adsorption, assuming that the adsorption takes place at specific homogeneous sites on the surface of the adsorbent and that all of the adsorption sites are energetically identical. The forces for adsorption include van der Waals forces, electrostatic interactions, π−π interactions, and hydrogen-bonding forces, with electrostatic interactions as the main force.49,50 For cellulose, the adsorption process involves multilayer adsorption, and the main forces, including van der Waals forces, hydrogen-bonding forces,38 and electrostatic interactions, are weak. 3.7. Desorption and Regeneration. Reusability, an important property of adsorbents, was studied through dynamic desorption experiments. In these experiments, 50% ethanol solution at pH 3.0 was used as the eluent to desorb CV. The results are summarized in Figures 8 and 9. Although there was a small decline in the first two adsorption/desorption cycles, the adsorption capability of CGS still remained high during the first eight cycles, which demonstrates that CGS is a regenerable, economical, and steady adsorbent for the removal of CV. As shown in Figure 9, the desorption efficiency was greater than

Figure 8. Adsorption of CV over several successive desorption/ adsorption cycles.

Figure 9. Desorption efficiency using 50% ethanol solution for CV over several successive desorption/adsorption cycles.

90% as the number of cycles increased, indicating that the employed ethanol solution is an ideal eluent. Because of the principle of compatibility between similar species, CV is easily dissolved in ethanol solution. In addition, H+ ions protonate the adsorbent surface in acidic environments, making regeneration of carboxyl groups (COOH) more favorable and electrostatic interactions weaker, so that the dye molecules can be more easily removed from CGS.4,44 In the first two adsorption/desorption cycles, adsorption was greater than in the following cycles, because of the presence of a few irreversible sites on CGS. With increasing number of cycles, the influence of the acid eluent (50% ethanol solution, pH 3.0) treatment accumulated, resulting in a decrease of the removal efficiency.51 3.8. Comparison with Other Adsorbents. To demonstrate the significance of this study, we performed a comparison of CGS with other adsorbents, as shown in Table 4. The adsorption capability of CGS was found to be greater than those of other reports adsorbents, which confirms that the new adsorbent CGS has potential for binding CV from aqueous solutions. 5504



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(5) Liu, Q. Q; Pan, C. Y. A Novel Route to Treat Wastewater Containing Cationic Dyes. Sep. Sci. Technol. 2012, 47, 630. (6) Wenger, S.; Schmid, M.; Rothenberger, G.; Gentsch, A.; Grätzel, M.; Schumacher, J. R. O. Coupled Optical and Electronic Modeling of Dye-Sensitized Solar Cells for Steady-State Parameter Extraction. J Phys Chem C 2011, 115, 10218. (7) Gharabaghi, M.; Irannajad, M.; Azadmehr, A. R. Selective Sulphide Precipitation of Heavy Metals from Acidic Polymetallic Aqueous Solution by Thioacetamide. Ind. Eng. Chem. Res. 2012, 51, 954. (8) Davezza, M.; Fabbri, D.; Prevot, A. B.; Pramauro, E. Removal of Alkylphenols from Polluted Sites Using Surfactant-Assisted Soil Washing and Photocatalysis. Environ. Sci. Pollut. Res. Int. 2011, 18, 783. (9) Silva Filho, E. C.; Lima, L. C.; Silva, F. C.; Sousa, K. S.; Fonseca, M. G.; Santana, S. A. Immobilization of Ethylene Sulfide in Aminated Cellulose for Removal of the Divalent Cations. Carbohydr. Polym. 2013, 92, 1203. (10) Nada, A.-A. M. A.; Hassan, M. L. Phosphorylated CationExchangers from Cotton Stalks and Their Constituents. J. Appl. Polym. Sci. 2003, 89, 2950. (11) Ashokkumar, M.; Narayanan, N. T.; Gupta, B. K.; Reddy, A. L. M.; Singh, A. P.; Dhawan, S. K.; Chandrasekaran, B.; Rawat, D.; Talapatra, S.; Ajayan, P. M.; Thanikaivelan, P. Conversion of Industrial Bio-Waste into Useful Nanomaterials. ACS Sustainable Chem. Eng. 2013, 1, 619. (12) Singh, K. P.; Gupta, S.; Singh, A. K.; Sinha, S. Optimizing Adsorption of Crystal Violet Dye from Water by Magnetic Nanocomposite Using Response Surface Modeling Approach. J. Hazard. Mater. 2011, 186, 1462. (13) Tang, Y. K.; Chen, L.; Wei, X. R.; Yao, Q. Y.; Li, T. Removal of Lead Ions from Aqueous Solution by the Dried Aquatic Plant, Lemna perpusilla Torr. J. Hazard. Mater. 2013, 244−245, 603. (14) Mittal, A.; Mittal, J.; Malviya, A.; Kaur, D.; Gupta, V. K. Adsorption of Hazardous Dye Crystal Violet from Wastewater by Waste Materials. J. Colloid Interface Sci. 2010, 343, 463. (15) Ren, Y.; Abbood, H. A.; He, F.; Peng, H.; Huang, K. Magnetic EDTA-Modified Chitosan/SiO2/Fe3O4 Adsorbent: Preparation, Characterization, and Application in Heavy Metal Adsorption. Cem. Eng. J. 2013, 226, 300. (16) Pitsari, S.; Tsoufakis, E.; Loizidou, M. Enhanced Lead Adsorption by Unbleached Newspaper Pulp Modified with Citric Acid. Cem. Eng. J. 2013, 223, 18. (17) Mahmoud, M. E.; Abdelwahab, M. S.; Fathallah, E. M. Design of Novel Nano-Sorbents Based on Nano-Magnetic Iron Oxide−BoundNano-Silicon Oxide−Immobilized-Triethylenetetramine for Implementation in Water Treatment of Heavy Metals. Cem. Eng. J. 2013, 223, 318. (18) Meder, F.; Hintz, H.; Koehler, Y.; Schmidt, M. M.; Treccani, L.; Dringen, R.; Rezwan, K. Adsorption and Orientation of the Physiological Extracellular Peptide Glutathione Disulfide on Surface Functionalized Colloidal Alumina Particles. J. Am. Chem. Soc. 2013, 135, 6307. (19) Pan, S. D; Zhang, Y.; Shen, H. Y; Hu, M. Q. An Intensive Study on the Magnetic Effect of Mercapto-Functionalized Nano-Magnetic Fe3O4 Polymers and Their Adsorption Mechanism for the Removal of Hg(II) from Aqueous Solution. Cem. Eng. J. 2012, 210, 564. (20) Fan, L. L.; Luo, C. N.; Sun, M.; Qiu, H. M.; Li, X. J. Synthesis of Magnetic β-Cyclodextrin−Chitosan/Graphene Oxide as Nanoadsorbent and Its Application in Dye Adsorption and Removal. Colloids Surf. B 2013, 103, 601. (21) Sancha, R.; Bajpai, J.; Bajpai, A. K. Designing of Fullers-EarthContaining Poly(vinyl alcohol)-g-poly(2-acrylamido-2-methyl-1-propanesulfonic acid) Nanocomposites: Swelling and Deswelling Behaviors. J. Appl. Polym. Sci. 2010, 118, 1230. (22) Dotto, G. L.; Vieira, M. L. G.; Pinto, L. A. A. Kinetics and Mechanism of Tartrazine Adsorption onto Chitin and Chitosan. Ind. Eng. Chem. Res. 2012, 51, 6862.

Table 4. Comparison with Other Adsorbents adsorbent jute fiber carbon (JFC) coniferous pinus bark powder (CPBP) TiO2-based nanosheet (TNS) sodium dodecylsulfate (SDS) palm kernel fiber (PKF) carAlg/MMt nanocomposite hydrogels opal magnetic nanocomposite cellulose CGS

dye

pH

qm (mg g−1)

CV CV

8 8.0

27.99 32.78

52 3

CV CV CV CV

8.5 7.0 8 6.4

58.3 76.9 78.9 88.8

53 54 47 55

CV CV CV CV

7.2 8.5 9.0 9.0

101.13 113.13 112 182.15

ref

2 12 this work this work

4. CONCLUSIONS This article has demonstrated that the new adsorbent CGS can be used as an effective adsorbent for the removal of CV from aqueous solutions. The modification of cellulose with SSA and GMA was found to effectively enhance its electronegativity, leading to an increase in adsorption capacity. The solution pH, time, temperature, and initial concentration were all investigated to determine the best experimental conditions. The adsorption kinetics and adsorption isotherm were also discussed. The experimental for of CGS fitted well to the PS-O kinetic model and the Langmuir isotherm model. The thermodynamic studies provided evidence for the feasibility and spontaneity of the adsorption process. Reusability tests showed that CGS can be effectively regenerated and successively reused for at least eight cycles.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-371-22862833-3422. Fax: +86-371-23881589. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the Key Scientific and Technological Project of Henan Province (112102310360, 122300410260), the Natural Science Research Project of the Henan Province Education Department (2011A610005), and the financial support of the Foundation of International Scientific and Technological Cooperation of Henan Province (124300510012) in China.

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REFERENCES

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Removal of Crystal Violet by a Novel Cellulose-Based Adsorbent

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