Sorption of methylene blue by carboxymethyl cellulose and reuse process in a secondary sorption

Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 143–151

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Sorption of methylene blue by carboxymethyl cellulose and reuse process in a secondary sorption夽 Han Yan a , Wenxuan Zhang a , Xiaowei Kan a , Lei Dong a , Ziwen Jiang a , Haijiang Li a , Hu Yang a,∗ , Rongshi Cheng a,b a Key Laboratory of Mesoscopic Chemistry of MOE, Department of Polymer Science and Technology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China b College of Material Science and Engineering, South China University of Technology, Guangzhou 510641, PR China

a r t i c l e

i n f o

Article history: Received 14 November 2010 Received in revised form 29 December 2010 Accepted 21 February 2011 Available online 5 March 2011 Keywords: Carboxymethyl cellulose Methylene blue Sorption behavior Secondary sorption Methyl orange

a b s t r a c t The sorption behaviors of carboxymethyl cellulose (CMC) for methylene blue (MB) were investigated in this work. The experimental results indicated that the sorption capacity increased from 50 mg g−1 for unmodified cellulose (UmC) to more than 300 mg g−1 for CMC. The most favorable sorption of MB was observed at an alkaline condition. The sorption isotherms closely followed the Langmuir mode, and the sorption kinetics was in agreement with the pseudo-second order equation. The results from the batch experiments illuminated that the sorption mechanism was ion-exchange controlled process. In fixed-bed tests, CMC also exhibited high efficiency for removal for MB, in which sorption behaviors followed Thomas model. Desorption of the dye from the MB-sorbed CMC (MBsCMC) indicated that MBsCMC was stable, and MB was seldom released at neutral and alkaline conditions. Furthermore, a more efficient method for reuse of the disused sorbents was tried. MBsCMC was employed for removal of methyl orange (MO) in a secondary sorption at neutral or alkaline conditions. The maximal MO uptake of MBsCMC was over 100 mg g−1 , which was much higher than those of CMC and UmC. It was indicated that MBsCMC was efficient in sorption of MO for the electrostatic interaction between MO and MBsCMC, and secondary sorption was an appropriate way for reuse of this kind of disused sorbents. © 2011 Elsevier B.V. All rights reserved.

1. Introduction With the increasing concern on environment protection in recent years, removal of pollutants is gaining public and technological attention. Dyes, which is the main component in the effluent of textile, paper, plastic, food and cosmetic industries, is an important category of contaminant in water bodies [1]. Dyes affect the environment in a number of aspects. They are highly visible and possess high light absorption, which affect the photosynthesis of aquatic plants. Many dyes are toxic and hazardous to aquatic organisms [2]. Moreover, most dyes are organic compounds of which the excess would increase the chemical oxygen demand (COD) of the water body [3]. Thus removal of dyes from wastewater becomes a significant issue. Currently, there are several methods for dye removal in wastewater treatment, such as sorption [4], flocculation [5], oxidation [6], and electrolysis [7]. Among these methods, sorption is gaining pop-

夽 Supported by the Key Natural Science Foundation of China (Grant nos. 50633030 and 51073077). ∗ Corresponding author. Tel.: +86 25 83686350; fax: +86 25 83686350. E-mail address: [email protected] (H. Yang). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.02.045

ularity due to its efficiency in dealing with various pollutants and the convenient procedure [4]. Nevertheless, conventional sorbents, such as activated carbon, still face a few of disadvantages. In spite of their effectiveness, many of them are quite expensive [8], and most of them are difficult to degrade in natural conditions, causing secondary pollutions. Therefore, in the past few years, biosorbents prepared from natural organisms are paid much more attentions. These sorbents, such as cellulose, chitosan, starch and lignin, are easily biodegradable and nontoxic. Nowadays there are a number of articles reporting the adsorptive removal of dye by these biosorbents [9–12], and most of them are experimentally successful and effective. Among these sorbents, cellulose, which is the most abundant natural polymer in nature [13], can be sufficiently obtained from agricultural wastes such as straw, and plant. It is low cost and environmental-friendly. Developing it into various kinds of sorbents is an appropriate method for both environmental protection and waste reuse. In fact, numerous reports have been made on the application of cellulose-based sorbents in wastewater treatment recently [14–18]. However, as the molecular structure of cellulose itself is compact and inactive, it is inefficient for dye removal without any modification. So proper treatment is required to introduce reactive sites and activate the sorption ability [19]. Currently, the

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N

N

overnight. At last, it was ground and sifted with a 100-mesh sieve, which was used as the sorbent in the following experiments. The yield of the products was around 60–70%.

S

N Cl

2.3. Materials characterization

-

(a) N

N

N

SO3

Na + (b) Scheme 1. The molecular structure of MB (a) and MO (b).

most widely adopted methods for modification of cellulose-based materials are included esterification [15], grafting [16–18], amination [17], etc. In this work, carboxymethylization has been chosen to modify cellulose, as this method was highly matured manufacturing procedure which has already been applied in industry, and had sufficient yield [20,21]. Methylene blue (MB) is a kind of cationic dyes, which may cause eye injury and nausea, vomiting, profuse sweating, etc. on ingestion [2]. MB is selected as one of the simulated dye pollutant, the molecular structure of which is given in Scheme 1. Furthermore, the fundamental sorption behaviors of CMC for removal of MB, including the effect of pH, sorption equilibrium, kinetics, and column experiment are investigated, respectively. However, after saturated sorption of MB, regeneration and reuse of the disused sorbent is also a problem in wastewater treatment. Conventionally, large amount of solvent as eluent, of which cost is not negligible, are employed to recover the sorbents for recycle use [8]. The effluents thus generated usually require further treatment to avoid the secondary pollution. In consideration of these disadvantages, a more efficient method for reuse of the disused sorbents have been tried in this work, that is, applying them in a secondary sorption for removal of another pollutant. In this method, no additional reagent for regeneration is necessary, and the potential of the disused sorbent is further exploited. In this case, after CMC adsorbed MB fully, it is employed for removal of methyl orange (MO), a kind of anionic dyes, which structure is also shown in Scheme 1. It is found that the MB loaded CMC shows high efficiency in sorption of MO for the electrostatic interaction between MO and the MB loaded CMC. 2. Experimental 2.1. Materials Cellulose (UmC) was kindly presented by Prof. Enyong Ding of South China University of Technology. Chloroacetic acid, hydrochloric acid (HCl), sodium hydrate (NaOH), ethanol and other reagents used in this work were all A.R. grade reagents. The deionized water was used in all experiments. 2.2. Preparation of carboxymethyl cellulose (CMC) [20,21] A desired amount of UmC was dispersed in NaOH alcohol–water solution, and chloroacetic acid was added dropwise into the mixture and left under agitation at 353 K for 2.5 h. The raw product of CMC was then washed by deionized water and dried at 343 K

2.3.1. FTIR spectra The FTIR spectra of UmC and CMC were obtained with a Bruker IFS 66/S IR spectrometer, respectively. All samples were prepared as potassium bromide tablets, and the scanning range was 650–4000 cm−1 . 2.3.2. SEM study After being sputter coated with gold, the surface morphologies of UmC and CMC were observed directly with a scanning electron microscope (Type SSX-550; Shimadzu Co.; Japan). The electron micrographs were taken with an acceleration voltage of 25.0 kV. 2.3.3. ␨ potential measurements A desired amount of CMC was dispersed in water under agitation for 24 h, followed by ultrasonic treatment for 30 min. The ␨ potential data was acquired from a Malvern Nano-Z ␨ potential recorder. The range of initial solution pH values was 1.0–11.0, adjusted by dilute H2 SO4 or NaOH aqueous solutions. 2.4. Sorption experiments for removal of MB 2.4.1. Sorption of MB at different initial pH of solution The influences of different initial pH to sorption of MB onto CMC and UmC were studied, respectively. The range of initial pH values of MB aqueous solutions was 0–12.0, adjusted by dilute H2 SO4 or NaOH aqueous solutions. The initial concentration of MB solutions was about 400 mg dm−3 . Sorbents were weighed and immerged into MB solutions with different pH values under continuous stirring at 298 K for 48 h to achieve sorption equilibrium. The initial and final MB concentrations were analyzed at a wavelength of 662 nm by a Victor 722 Vis Spectrometer. Appropriate dilution was processed to ensure that the concentration of the solution was within the range of the standard curve. The amount of sorption in batch experiments, q (mg g−1 ), was calculated according to the following equation: q=

(C0 − Ce )V m

(1)

where C0 and Ce (mg dm−3 ) are the initial and final concentrations of solute in solution, respectively; V (dm3 ) is the volume of solution; and m (g) is the mass of the sorbent. 2.4.2. Sorption equilibrium study The sorption equilibrium study was conducted at different temperatures: 293, 298, 303, and 313 K, respectively. The concentrations of MB aqueous solutions ranged from 10 to 1000 mg dm−3 . A desired amount of CMC was weighed and dosed in each of the MB solutions under continuous stirring for 48 h at initial pH 7.0. The sorption behaviors of UmC were also carried out at 298 K for comparison. The same analysis method as mentioned in previous part has been employed to detect the initial and final MB concentrations by Vis Spectrometer. The amount of sorption was calculated based on Eq. (1). 2.4.3. Sorption kinetics study The kinetic sorption experiments were also measured at varied temperatures: 293, 298, 303, and 313 K, respectively. The initial concentration of MB solutions and pH value were fixed at 400.0 mg dm−3 and 7.0, respectively. CMC was weighed and immerged into MB solutions under continuous stirring at varied

H. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 143–151

temperatures. Then, 1 cm3 of sample solutions was taken out at desired time intervals to analyze the current MB concentration. Meanwhile, the same volume of water with pH 7.0 was added into the bulk solutions to keep the volume constant. The sorption amount of MB at time ti , q(ti ) (mg g−1 ), was calculated from the following equation: q(ti ) =

(C0 − Cti )V0 −

i−1 2

Cti−1 Vs

m

 Nt 0

CB dNt

m

0

1598

b

1426 1730

a

(2)

2.4.4. Column experiments For real applications, column experiments were conducted in a glass column (12.8 mm in diameter and 200 mm in length) equipped with water baths to keep the test temperature constant. The mass of sorbent was 1.02, 1.51, and 2.01 g, with corresponding bed depth of 4.50, 6.81, and 9.30 cm, respectively. The concentration of MB solutions was 2040.3 mg dm−3 . Temperature and initial solution pH were kept at 298 K and 7.0, respectively. The flow rate was 1.20 cm3 min−1 . The amount of sorption in the fixed-bed study, qf (mg g−1 ), was calculated according to the equation given below: C0 VB Ne − VB

c

T%

where C0 and Cti (mg dm−3 ) are the initial MB concentration and MB concentrations at time ti (min), respectively. V0 and Vs (dm3 ) are the volume of the MB solution and that of the sample solution taken out every time for MB concentration analysis, respectively. Here, Vs is equal to 0.001 dm3 . And m is the mass of sorbent (g).

qf =

25

145

3300

1050

4000

3000

2000

Wavenumber ( cm -1 )

1000

Fig. 1. FTIR spectra of UmC (a), H-form CMC (b), and Na-form CMC (c).

the MBsCMC layer. The initial concentration of MO and solution pH value were 496.1 mg dm−3 and 7.0, respectively. The sorption behaviors of UmC and CMC for removal of MO in column study were also carried out under similar conditions for comparison. 3. Results and discussion 3.1. Preparation of the sorbents

(3)

where C0 and CB (mg dm−3 ) are the initial MB concentration, and MB concentrations at a certain bed volume number, respectively; VB (cm3 ) is the bed volume; Nt is the bed volume number at time t (min); Ne is the bed volume number after reaching sorption equilibrium; and m (g) is the mass of sorbent. 2.5. Secondary sorption study for removal of MO 2.5.1. Desorption studies After saturated sorption of MB, the MB-adsorbed CMC (MBsCMC) was collected and washed by deionized water. The dye-loaded sorbent was then dried at 343 K. The effects of different initial pH on MB desorption from CMC were studied at room temperature. MBsCMC were weighed, and then immerged into different aqueous solutions with various pH values ranged from 0 to 12.0 under continuous stirring for 48 h. The final MB concentrations in solution were analyzed based on aforementioned method Eq. (1) to estimate the amount of desorption. 2.5.2. Sorption of MO at different initial pH The collected MBsCMC was washed and dried at 343 K, which was applied as a new sorbent in secondary sorption experiments for removal of MO. The influences of different initial pH to sorption of MB onto MBsCMC, CMC and UmC were studied at 298 K, respectively. The detailed process was similar to that of MB as described in Section 2.4.1. The range of initial pH values of MO aqueous solutions was 6.0–12.0. The initial concentration of MO solutions was about 500 mg dm−3 . The initial and final MO concentrations were determined at a wavelength of 464 nm by a Victor 722 Vis Spectrometer. The amount of sorption was calculated based on Eq. (1). 2.5.3. Column study Furthermore, column study had been also done for removal of MO by the disused sorbents of MBsCMC. The detailed process was similar to that of MB as described in Section 2.4.4. The mass of MBsCMC was 1.01 g. For sorption of the available desorbed MB from MBsCMC, a small amount of CMC was loaded in the column under

CMC has been prepared according to reported method [20,21], and the detailed preparation processes for CMC were described in Section 2. The FTIR spectra of UmC and CMC were shown in Fig. 1, respectively. From Fig. 1, typical peaks of cellulose were observed in the spectra of cellulose and its derivatives. The wide peak at about 3300 cm−1 could be attributed to the stretching vibrations of –OH groups [22]. The intense peak around 1050 cm−1 corresponded to C–O–C bonds [23]. For further investigation of the carboxyl groups on CMC, H-form and Na-form CMC have been prepared in acidic and alkaline solutions for FTIR measurement, respectively. In the FTIR spectrum of H-form CMC as shown in Fig. 1b, a new characteristic peak appeared at 1730 cm−1 , which was assigned to –COOH. However, in the spectrum of CMC–Na in Fig. 1c, only symmetric and asymmetric stretching at 1598 cm−1 and 1426 cm−1 , respectively, were observed [24,25]. It was indicated that carboxyl groups has been grafted onto cellulose backbone, and CMC had been prepared successfully. The SEM images of UmC and CMC were shown in Fig. 2, respectively. It was found that the surface morphology of CMC was quite similar with that of UmC, which indicated that this reaction did not affect the surface morphology of cellulose evidently. 3.2. Sorption of MB 3.2.1. Effect of initial pH and ␨ potential on sorption of MB CMC and UmC had been both employed for removal of MB from aqueous solutions. The effects of initial pH on MB sorption were investigated at the beginning, since pH was one of very important factors for ion-type sorbents. Fig. 3 shows the pH dependence of CMC and UmC for removal of MB, respectively. It was found that the sorption capacity of UmC was nearly independent of pH, and always kept at a quite low level, since UmC was a type of nonionic and inactive sorbents. However, as for CMC, at lower pH, the sorption capacity was quite similar to that of UmC. But it increased rapidly with initial pH increase until pH reached optimal one, then decreased. The highest sorption capacity of CMC at pH around 10.0 was 331.5 mg g−1 . It was indicated that the increase of sorption capacity of CMC should

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Fig. 2. SEM images of UmC (a) and CMC (b).

be ascribed to the additional carboxyl groups. In acidic conditions, the carboxyl groups of CMC were protonized. Furthermore, based on the ␨ potential variation of CMC as also shown in Fig. 3, ␨ potential of CMC was positive at lower pH, and the isoelectric point was about 2.0. Therefore, CMC in strong acidic condition had very low affinity with cationic MB, and low sorption capacity. Besides, the decomposition of the cellulose chain in strong acidic conditions may partly influence the sorption capacity also. With pH increase, ␨ potential of CMC turned negative. At higher pH, the carboxyl groups were fully deprotonized, and CMC showed higher sorption capacity. However, when pH increased further, precipitation of MB with excessive hydroxyl ions would occur simultaneously, and could not accurate interpretation of sorption. The MB uptakes of CMC decreased at pH higher than 10.0. Furthermore, in comparison with other biosorbents, such as EDTAD-modified sugarcane bagasse [1] and rejected tea [26], CMC showed higher efficiency in removal of MB. Recently, Yu et al. [27] reported a poly(methacrylic acid) modified biomass, which showed much greater maximal MB uptake around 869.6 mg g−1 . They thought that it was due to more carboxyl groups introduced on the biomass surface, and the sorption capacity was enhanced [27]. On the other hand, it further supported that carboxyl group played a key role in removal of MB. 3.2.2. Isothermal sorption equilibrium study For further investigation of the interactive behavior between the solutes and sorbents, the sorption isotherms of MB on CMC at different temperatures as well as that of UmC were measured at pH around 7.0, and shown in Fig. 4, respectively. According to Fig. 4, it was found that the sorption capacity of CMC was affected insignificantly by temperature and showed

temperature independent at neutral condition. In addition, the equilibrium uptakes of MB on CMC was about three to four times greater than that of UmC, which indicated that carboxyl groups on CMC enhanced the sorption capacity greatly. Furthermore, for study on the sorption mechanism, the isotherms data were analyzed in detail on the basis of Langmuir, Freundlich, Sips, and Dubinin–Radushkevich (D–R) models, respectively. The Langmuir isotherms model was based on an idealized assumption of identical sorption heat and monolayer sorption [28]. The linear form of Langmuir equation was given as follows: Ce 1 = + qe qL bL

(4)

Ce

ln qe = ln KF +

ln Ce n

(5)

where qe (mg g−1 ) is the sorption capacity at equilibrium; Ce (mg dm−3 ) is the concentration of the MB solution at equilibrium; KF is the Freundlich isotherm constant; and 1/n is the heterogeneity factor.

250

0

-10

150 75

-20

0 0

2

4

6

pH

8

10

12

Fig. 3. Effects of initial pH on MB sorption by various sorbents: CMC () and UmC (), and the pH dependence of ␨ potential variation of CMC ().

-1

ζ potential

225

200

qe (mg g )

300 -1

qL

where qe (mg g−1 ) is the sorption capacity at equilibrium; Ce (mg dm−3 ) is the concentration of the MB solution at equilibrium; qL (mg g−1 ) is the theoretical saturate sorption capacity; and bL (dm3 mg−1 ) is a constant reflecting the tendency of sorption. The Freundlich model was an empirical model for heterogeneous system, and it described reversible sorption and was not restricted to the formation of the monolayer. Freundlich model successfully interpreted a number of sorption phenomena in gas phase or solutions [29]. The following equation expressed it in a linear form.

375

qe (mg g )

1

150 100 50 0

200

400

600

800

-1

1000 1200

C0 (mg g ) Fig. 4. Isotherms of MB sorption on CMC at various temperatures: 293 K (), 298 K (), 303 K () and 313 K (♦), and the comparative isotherm of UmC () at 298 K.

12.28 12.45 12.69 12.63 10.16 3.316 3.225 3.103 3.132 4.810 0.9755 0.9657 0.975 0.9672 0.9054 1.3010 1.1553 1.1132 0.9931 1.4232 0.1672 0.1302 0.1375 0.1127 0.0376 247.7 228.1 222.5 216.7 65.55 0.826 0.8362 0.8238 0.8256 0.8042 2.667 2.667 2.69 2.252 2.051 31.50 29.07 29.51 23.33 2.705 0.03002 0.02371 0.01688 0.02710 0.1657 0.9935 0.9986 0.9962 0.9971 0.9777 0.0793 0.1028 0.1454 0.0896 0.0126 244.5 222.2 215.0 219.8 57.21 a

UmC

C0 ≈ 400 mg dm−3 .

223.4 214.2 209.6 209.1 47.16 293 298 303 313 298

RL a

n

Freundlich model

where C0 (mg dm−3 ) is the initial concentration of MB solutions. The value of RL indicated the tendency of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0) [33]. Smaller RL values expressed greater affinity between the sorbent and the solutes. From Table 1, the RL values of CMC and UmC were estimated, respectively, and C0 was 400 mg dm−3 , at which a saturated sorption was achieved for the sorbents. It was found that all RL were smaller than 1.0, while the RL values of CMC were much smaller than that of UmC, which was indicative of the enhancement in sorption capacity after modification. Meanwhile, according to the analysis based on the Sips model, of which the correlation coefficients were higher than 0.95, all of the calculated 1/s for CMC at varied temperatures were relatively close to 1. It indicated that CMC was a homogeneous sorbent, not a heterogeneous one described by Freundlich model. In addition, from Table 1, the lower R2 values of Freundlich model supported above discussion further. Moreover, according to the analysis by D–R model, the mean energy of sorption (E) could be obtained. The value of E would give information about sorption mechanisms: the sorption process could be described as a physical one when E was between 1.0 and

KF

(9)

R2

1 (1 + bL C0 )

Table 1 The parameters from the simulated results based on various isothermal models.

RL =

R2

where K is the constant related to the mean free energy of sorption. The simulated parameters by aforementioned models were all listed in Table 1. Based on the correlation coefficients (R2 ) of the linear form for the various models, the R2 of Langmuir model was much closer to 1 than that of other models. Furthermore, from Table 1, the qL of CMC calculated from Langmuir model at varied temperatures were all close to their experimental values (qex ). It was indicated that the sorption behaviors of MB on CMC were described much better by Langmuir model, and MB was in monolayer coverage on the CMC surface. In addition, from the constant bL , the dimensionless constant of RL was calculated further, which was employed to describe the essential characteristic of the Langmuir model [33]:

CMC

E (kJ) D-R model

s bs (dm−3 mg−1 )

(8)

b (dm3 mg−1 )

E = (2K)−1/2

Sips model

where qe (mg g−1 ) is the sorption capacity at equilibrium, K is the constant related to the mean free energy of sorption, qD (mg g−1 ) is the theoretical saturation capacity in this model, and ε is the Polanyi potential (ε = RT ln(1 + (1/Ce ))). Information regarding the mean energy of sorption (E), can be obtained by the following equation:

qS (mg g−1 )

(7)

Langmuir model

ln qe = ln qD − Kε2

R2

where qe (mg g−1 ) is the sorption capacity at equilibrium; bs (mg dm−3 ) is the median association constant; qS (mg g−1 ) is the theoretical saturated sorption capacity based on this model; Ce (mg dm−3 ) is the concentration of MB solution at equilibrium; s reflects the heterogeneity of the sorbent. The heterogeneity factor of 1/s  1 indicates heterogeneous sorbents, while values close to or even 1.0 indicate materials with relatively homogenous binding sites, in which case Sips model can be reduced to Langmuir equation [31]. The D–R isothermal model was employed for determination of the nature of biosorption processes [32]. The linear equation of D–R model was given below:

K × 109

(6)

qL (mg g−1 )

1 + qS Ce 1/s

qex (mg g−1 )

bS qS Ce 1/s

Samples T (K)

qe =

R2

Furthermore, Sips model was considered as a combination of Langmuir and Freundlich equations [30]. The following equation gave Sips model for sorption in solutions:

147

0.8835 0.8970 0.8869 0.8802 0.7910

H. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 143–151

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H. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 143–151

200

COO- Na+

+

MB+

COO- MB+

+

Na+

-1

qt (mg g )

160 Scheme 2. Ion-exchanging mechanism of MB sorption on CMC, MB+ represents the cationic part of MB.

120 80

Its expression was presented below [37]:

40

qt = AE + BE ln t

0

10

20 t (min)

30

qt (mg g−1 ) is the sorption capacity at a certain time t (min); AE and BE are both the Elovich constants. The simulated parameters by aforementioned kinetic models were all listed in Table 2. Based on the analysis of correlation coefficients, it was found that the sorption kinetics of CMC were in excellent agreement with the pseudo-second order kinetic model, further confirming that the sorption was a chemical sorption. In addition, Elovich equation also fitted the experimental data well, and R2 was higher than 0.9, which indicated that the sorption behaviors of MB on CMC were ion exchange reactions. Thus, the sorption mechanism of MB on CMC may be described in Scheme 2. Furthermore, on the basis of the pseudo-second order kinetics, the sorption rate constants of k2 can be obtained from the slope of the lines. Moreover, the activation energy can be calculated according to the Arrhenius equation:

40

Fig. 5. Kinetic results of MB sorption on CMC at various temperatures: 293 K (), 298 K (), 303 K () and 313 K (♦).

8.0 kJ mol−1 , or a chemical one when E was more than 8.0 kJ mol−1 [34]. From Table 1, the E values derived in this analysis was around 12–13 kJ mol−1 , which was primarily indicative of a chemical sorption of MB on CMC. 3.2.3. Sorption kinetics Then, sorption kinetics had been also carried out to explore the sorption mechanisms further at pH around 7.0. Fig. 5 showed the q–t curves of CMC at different temperatures. It was found that the sorption equilibrium time for CMC showed temperature independent. In addition, the sorption process was quite fast, in which sorption equilibrium was achieved less than half an hour at various temperatures. The fast sorption rate suggested that CMC was an effective sorbent and had strong interaction with solutes. Next, the pseudo-first order, pseudo-second order and Elovich kinetic models were employed, respectively, for analyzing the experimental data for further discussion. The pseudo-first order model, which was developed by Langeren, assumed that the sorption rate decreased linearly as the sorption capacity increased [35]. It was given as the equation below: ln(qe − qt ) = ln qe − k1 t

k2 = AA exp

 −E  a

(13)

RT

where AA is the Arrhenius constant; Ea (J mol−1 ) is the activation energy; R (J mol−1 K−1 ) is the gas constant; T (K) is the temperature. The value of activation energy can provide information on the mechanism of the ion exchange process. In general, the sorption process was classified to be film-diffusion controlled when Ea was below 16 kJ mol−1 , particle-diffusion controlled when Ea was 16–40 kJ mol−1 , and chemical-reaction controlled when Ea was greater than 40 kJ mol−1 [38,39]. The activation energy of MB sorption was 23.6 kJ mol−1 , and it suggested that the sorption of MB on CMC was particle-diffusion controlled process.

(10)

where qe and qt (mg g−1 ) are the sorption capacity at equilibrium and that at a certain time t (min), respectively; k1 (min−1 ) is the pseudo-first-order rate constant. Pseudo-second order kinetic model assumed that the ratelimiting step was the interaction between two reagent particles [36]. Its linear form was given as the following equation: 1 t t = + qt qe k2 qe 2

(12)

3.2.4. Fixed-bed experiments For real application, the column experiments were carried out. The breakthrough curves of MB sorption on CMC at various bed depths were presented in Fig. 6. CMC exhibited effective sorption ability, and almost completely adsorbed MB from influent before the breakthrough point. Based on Eq. (3), the amount of MB adsorbed in column qf was calculated, and showed in Table 3, which was much higher than that of UmC. In addition, it seemed that the MB uptakes were affected by the amount of sorbents in column. At the beginning, the MB uptakes increased with the bed depth increase until 6.8 cm; then there was no significant change as increased further. It might be ascribed to that the contact time was long enough to achieve sorption equilibrium after the bed depth was higher than 6.8 cm. The theoretical analyses of fixed-bed experiments in MB sorption and the following secondary sorption were both based

(11)

where qe and qt (mg g−1 ) are the sorption capacity at equilibrium and that at a certain time t (min), respectively; k2 (g mg−1 min−1 ) is the pseudo-second order rate constant. Elovich model was used to express the kinetics of chemical sorption of gas on a heterogeneous surface, conventionally. While the application of Elovich equation in liquid phase to describe the sorption kinetics of ion exchange process was now gaining in popularity. Table 2 The parameters from the simulation results based on various kinetic models. Samples

T (K)

Pseudo-first order model −1

k1 (min CMC

293 298 303 313

0.1280 0.1609 0.2311 0.3115

)

2

Pseudo-second order model

R

k2 (g mg

0.7774 0.8104 0.7171 0.4222

0.0086 0.0104 0.0120 0.0161

−1

min

−1

)

Elovich model R

AE (mg g−1 )

BE (mg g−1 )

R2

0.9985 0.9975 0.9855 0.9973

123.2 130.1 104.0 125.9

17.3 19.5 29.2 22.2

0.9487 0.9180 0.9490 0.9223

2

0.9022 0.9207 0.8402 0.5742 0.9257 0.3557 0.3247

1.0

C/C t 0

89.8 143 188 19.0 281 10.3 11.1

0.8 0.6 0.4 0.2 0.0 25

30

0.9945 0.9970 0.9959 0.9872 0.9922 0.9763 0.9943 189 237 217 20.8 149 4.31 3.61

3.74 4.47 4.21 2.04 4.03 0.209 0.285

0.7649 0.8304 0.8118 0.4653 0.7879 0.5221 0.2868

20

Fig. 6. The fixed-bed breakthrough curves of MB sorption by CMC with different bed heights: 4.50 cm (), 6.81 cm (), and 9.30 cm (), and the comparative experiment of UmC () at 298 K.

194 236 220 23.0 146 4.66 4.03 1.02 1.51 2.01 1.50 1.01 1.00 1.02 MO

MB

UmC MBsCMC UmC CMC

4.50 6.81 9.30 3.83 4.01 2.71 4.52

C  t

C0

= kAB C0 nN − kAB Q

Z U

(15)

where kAB (dm3 min−1 mg−1 ) is the Adams–Bohart rate constant, Q (mg dm−3 ) is the theoretical saturated sorbate concentration in sorbent in the Adams–Bohart model, U (cm min−1 ) is the flow rate of the effluent, n (min) is the BV-time conversion factor, N is the bed volume number, C0 (mg dm−3 ) is the influent MB concentration, Ct (mg dm−3 ) is the effluent concentration at time t. Moreover, the Yoon–Nelson model was a relatively simple model which assumed that the rate of decrease in the probability of sorption for each sorbate molecule was proportional to the probability of adsorbate sorption and the probability of adsorbate breakthrough on the adsorbent [43].



ln

CMC

(14)

where kTh (cm3 min−1 mg−1 ) is the Thomas rate constant, qTh (mg g−1 ) is the theoretical saturated sorption capacity in Thomas model, f (cm3 min−1 ) is the flow rate of the effluent, m (g) is the mass of the sorbent, VB (cm3 ) is the effluent volume, C0 (mg cm−3 ) is the influent MB concentration, Ct (mg cm−3 ) is the effluent concentration at time t and N is the bed volume number. The value of Ct /C0 is the ratio of effluent and influent MB concentrations at certain time. The Adams–Bohart model assumed that the sorption rate was proportional to both the residual capacity of the sorbent and the concentration of the sorbate species [42]. ln

0.0589 0.0482 0.0484 0.5661 0.2096 0.8132 0.9708

Adams-Bohart model

15

effluent, BV

6.66 4.10 3.25 0.923 4.11 82.7 26.0

R2 qTh (mg g−1 ) kT (cm3 mg −1 min−1 )

10

Ct 1 = C0 1 + exp((kTh /f )(qTh m − C0 VB N))

qf (mg g−1 ) Bed Depth (cm) Sample Dyes

5

on Thomas, Adams–Bohart and Yoon–Nelson model, respectively. The Thomas model assumed a Langmuir model of sorptiondesorption process and no axial dispersion was derived with the sorption that the rate driving force obeyed second-order reversible reaction kinetics [40,41].

m (g)

Thomas model

kAB × 105 (dm3 mg −1 min−1 )

Q/104 (mg dm−3 )

R2

0 Table 3 The parameters in fixed-bed experiments for sorption of various dyes by different sorbents with various bed depths according to Thomas, Adams-Bohart and Yoon-Nelson models.

149

1.2

0.125 0.0918 0.0696 0.180 0.0289 0.1244 0.0680

 (min) kYN (min−1 )

Yoon-Nelson model

R2

H. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 143–151

Ct C0 − Ct



= kYN nN − kYN 

(16)

where kYN (min−1 ) is the Yoon–Nelson rate constant, n (min) is the time-BV Scale factor, N is the bed volume number,  is the time required for 50% sorbate breakthrough (min), C0 (mg dm−3 ) is the influent MB concentration, Ct (mg dm−3 ) is the effluent concentration at time t. The simulated results by aforementioned models were all listed in Table 3. According to the correlation coefficients of the linear form for various models, a more accurate prediction could be made on the sorption process of CMC in fixed-bed systems by Thomas model. Furthermore, from Table 3, it was found that the theoretical saturated sorption capacity (qTh ) derived from Thomas model were

150

H. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 143–151

175

140

150 120 -1

qe (mg g )

100 75

100 20

50

10

25

0

0

0

2

4

6

pH

8

10

6

12

Fig. 7. Effects of initial pH on the MB desorption from MBsCMC.

almost the same as the experimental values. The accordance of the experimental data with the Thomas model also indicated that the sorption behavior of MB on CMC was a Langmuir-type sorption and followed pseudo-second order kinetics, which was fully consistent with the analyses in the batch studies. 3.3. Secondary sorption of MO onto MBsCMC 3.3.1. Desorption experiments It was well known that it was very important to regenerate the disused sorbents in practice applications. As mentioned above, the traditional treatments for recycle of those sorbents were not economical enough for application of large amount of solvent as eluent. In this case, the MB loaded CMC was not recovered as usual, but used as a new sorbent for removal of MO, a kind of anionic dyes. However, before this application, the stability of the new sorbents should be checked firstly. The pH dependence of MB desorption from MBsCMC were shown in Fig. 7. It was found that large amount of MB was released in strong acidic conditions, which was in accordance with previous MB sorption study. While the desorption behaviors became insignificant as pH increased. As Fig. 7 suggested, MBsCMC was stable enough after pH was higher than 4.0, which could be used as a new sorbent for secondary sorption. 3.3.2. Effect of initial pH on the secondary sorption of MO The effects of initial pH on MO sorption by MBsCMC, UmC and CMC, were shown in Fig. 8, respectively. It was found that MBsCMC was effective in the secondary sorption for removal of MO at pH ranged from 6.0 to 12.0, and the MO uptakes of MBsCMC was over 100 mg dm−3 . While UmC and CMC exhibited very low sorption capacities, and there was no significant change in MO uptakes within the measured pH range. It was due to the different surface structure of CMC before and after MB loaded. The anionic MO had good affinity with MB loaded CMC for electrostatic interaction. MO was closely connected with CMC through the cationic MB. However, a slight decrease tendency of MO uptakes

7

8

9

pH

10

11

12

Fig. 8. Effects of initial pH on secondary sorption for MO on MBsCMC (), UmC () and CMC ().

1.0 0.8

C/t C0

-1

Cd (mg g )

125

0.6 0.4 0.2 0.0 0

20

40

60

effluent, BV

80

100

Fig. 9. The fixed-bed breakthrough curves of MO sorption by MBsCMC (), UmC () and CMC () at 298 K.

for MBsCMC was observed as pH increased, which may resulted from the competition of the active sorption sites by excessive OH− ions. In addition, MO and MB had similar hydrophobic structure as shown in Scheme 1, which may result in MO/MB interaction for the hydrophobic effect. However, since the sorption behavior was pH dependent, the electrostatic interaction between MO and MB loaded CMC was more significant than hydrophobic interaction. Moreover, on the basis of the sorption capacity (expressed in mmol g−1 ), it can be calculated that the MB uptake (0.65–0.78 mmol g−1 ) on CMC was approximately twice than MO uptake (0.33–0.45 mmol g−1 ) on MBsCMC. It suggested that one MO ion may be trapped by two MB ions, which was ascribed to no sufficient positive charge for one MB ion to attract one MO ion, since MB was binding on CMC. Scheme 3 describes the MO adsorption mechanism by MBsCMC. MB as a bridge linked MO onto CMC. 3.3.3. Fixed-bed studies Next, column experiment has been also conducted at pH around 7.0. Fig. 9 shows the breakthrough curves of MO sorption on

Scheme 3. A possible mode of MO binding on MBsCMC due to electrostatic attractions.

H. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 143–151

MBsCMC, UmC and CMC, respectively. Neither UmC nor CMC showed any significant MO uptakes. The sorption capacity of CMC was even worse than UmC for electrostatic repulsion. In contrary to UmC and CMC, MBsCMC exhibited an effective sorption in the fixed-bed system, as MO in the influent has been adsorbed completely before the breakthrough point. Besides, no MB was observed in the effluent during the experiment, confirming that MBsCMC was stable enough at neutral condition. Furthermore, the simulated results for the breakthrough curves based on Thomas, Adams–Bohart and Yoon–Nelson models were also listed in Table 3. It was found that Thomas model described the breakthrough curves much more properly for higher correlation coefficient. In addition, the simulated MO uptakes (qTh ) were also very close to the experimental values calculated from Eq. (3). It indicated that MO sorption mechanism by MBsCMC followed monolayer chemical sorption. 4. Conclusions According to the experimental results in both batch and column studies as mentioned above, it was found that CMC was an effective sorbent for removal of MB from aqueous solutions. The experimental results showed that MB sorption was pH dependent, but temperature independent. Sorption isotherms were best fitted by Langmuir model, while the kinetics behaviors were well described by pseudo-second order kinetic model. The sorption rate of MB onto CMC was very fast reaching to sorption equilibrium less than half an hour. Thomas model was suitable to describe MB sorption in a fixed-bed system. It was indicated that the sorption behavior was a monolayer chemical sorption with an ion exchange process. In addition, a new efficient and economical method to treat with these disused sorbents has been tried in this paper. After saturated sorption of MB, the MB loaded CMC were used as a new sorbent for removal of MO from aqueous solutions in secondary sorption at neutral or alkaline conditions. The high MO uptake was ascribed to electrostatic interactions. References [1] Y. Xing, D. Liu, L.P. Zhang, Enhanced sorption of methylene blue by EDTADmodified sugarcane bagasse and photocatalytic regeneration of the sorbent, Desalination 259 (2010) 187–191. [2] B.H. Hameed, A.T.M. Din, A.L. Ahmad, Sorption of methylene blue onto bamboobased activated carbon: kinetics and equilibrium studies, J. Hazard. Mater. 141 (2007) 819–825. [3] D. Mantzavinos, E. Psillakis, Enhancement of biodegradability of industrial wastewaters by chemical oxidation pre-treatment, J. Chem. Technol. Biotechnol. 79 (2004) 431–454. [4] M. Rafatullah, O. Sulaiman, R. Hashim, A. Ahmad, Sorption of methylene blue on low-cost sorbents: a review, J. Hazard. Mater. 177 (2010) 70–80. [5] R. Fang, X. Cheng, X. Xu, Synthesis of lignin-base cationic flocculant and its application in removing anionic azo-dyes from simulated wastewater, Bioresour. Technol. 101 (2010) 7323–7329. [6] M. Dukkanci, G. Gunduz, S. Yilmaz, R.V. Prihod’ko, Heterogeneous Fenton-like degradation of rhodamine 6G in water using CuFeZSM-5 zeolite catalyst prepared by hydrothermal synthesis, J. Hazard. Mater. 181 (2010) 343–350. [7] X.C. Ruan, M.Y. Liu, Q.F. Zeng, Y.H. Ding, Degradation and decolorization of reactive red X-3B aqueous solution by ozone integrated with internal microelectrolysis, Sep. Purif. Technol. 74 (2010) 195–201. [8] D.W. O’Connell, C. Birkinshaw, T.F. O’Dwyer, Heavy metal sorbents prepared from the modification of cellulose: a review, Bioresour. Technol. 99 (2008) 6709–6724. [9] S. Mahesh, G.V. Kumar, P. Agrawal, Studies on the utility of plant cellulose waste for the biosorption of crystal violet dye, J. Environ. Biol. 31 (2010) 277–280. [10] H.Y. Zhu, R. Jiang, L. Xiao, W. Li, A novel magnetically separable gammaFe2 O3 /crosslinked chitosan sorbent: preparation, characterization and sorption application for removal of hazardous azo dye, J. Hazard. Mater. 179 (2010) 251–257. [11] Z. Wang, B. Xiang, R. Cheng, Y. Li, Behaviors and mechanism of acid dyes sorption onto diethylenetriamine-modified native and enzymatic hydrolysis starch, J. Hazard. Mater. 183 (2010) 224–232.

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