Adsorption of anionic dyes from aqueous solutions using chemically modified straw

Adsorption of anionic dyes from aqueous solutions using chemically modified straw

Bioresource Technology 117 (2012) 40–47 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com...

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Bioresource Technology 117 (2012) 40–47

Contents lists available at SciVerse ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Adsorption of anionic dyes from aqueous solutions using chemically modified straw Wenxuan Zhang a, Haijiang Li a, Xiaowei Kan a, Lei Dong a, Han Yan a, Ziwen Jiang a, Hu Yang a,⇑, Aimin Li a, Rongshi Cheng a,b a b

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, School of Chemistry & Chemical Engineering, Nanjing University, Nanjing 210093, PR China Polymer Institute, College of Material Science and Engineering, South China University of Technology, Guangzhou 510640, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" A quaternary ammonium cationic

The cationic modified wheat straw adsorbent showed high removal efficiency for two anionic dyes. Then, the anionic dyes loaded adsorbents were successfully applied to eliminate a cationic dye in the secondary adsorption.

modified straw (MWS) was prepared. " The fundamental adsorption behavior of MWS for two anionic dyes was studied. " They followed a monolayer chemical adsorption mechanism with ion exchange process. " The used adsorbent was effective in removal of cationic dye in secondary adsorption.

a r t i c l e

i n f o

Article history: Received 28 February 2012 Received in revised form 18 April 2012 Accepted 18 April 2012 Available online 25 April 2012 Keywords: Cationic modified straw Dye adsorption Column study Secondary adsorption Adsorption mechanism

a b s t r a c t The effective disposal of redundant straw is a significant work for environmental protection and full utilization of resource. In this work, the wheat straw has been modified by etherification to prepare a kind of quaternary ammonium straw adsorbents. The adsorption behaviors of the modified straw for methyl orange (MO) and acid green 25(AG25) were studied in both batch and column systems. The adsorption capacity of the straw for both dyes improved evidently after modification. The maximal MO and AG25 uptakes were more than 300 and 950 mg g1, respectively. Furthermore, the adsorption equilibrium, kinetics and column studies all indicated that the adsorption behavior was a monolayer chemical adsorption with an ion-exchange process. In addition, after adsorption of anionic dyes, the used adsorbents were successfully applied to adsorb a cationic dye directly at suitable conditions in the secondary adsorption. This was due to the altered surface structures of the used adsorbents. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, with the rapid development of modern industries, the environmental contamination associated with the dyes present ⇑ Corresponding author. Tel.: +86 25 83686350; fax: +86 25 83317761. E-mail address: [email protected] (H. Yang). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.04.064

in wastewater of various industrial sections, such as dyeing, printing, textile, leather, and coating industries, has drawn much attention. It is estimated that more than 70,000 tones of dyes are discharged in effluent from textile and associated industries in the world every year. The release of dyes has posed serious environmental problems. Colored dye effluents may interfere with light penetration in the receiving water bodies thereby disturbing the

W. Zhang et al. / Bioresource Technology 117 (2012) 40–47

biological processes. In addition, some dyes degrade into compounds that have toxic influences on mammals and aquatic organisms. Diverse techniques, including adsorption, flocculation, oxidation and electrolysis, have been employed for removal of dyes from wastewaters. Among these methods, adsorption is superior since it is more efficient and economical than others (Ngah et al., 2011). Activated carbon, the commonly used adsorbent, is effective in eliminating dyes but comparatively costly. Therefore, many biological materials, especially agricultural residues including peanut hull (Tanyildizi, 2011), sugarcane bagasse (Yu et al., 2011), coir pith (Khan et al., 2011), and maize cob (Sonawane and Shrivastava, 2009), have been investigated for removal of dyestuff in recent years. As a type of reproducible crop wastes, straw materials are not only abundant in nature but rich in available reactive groups existing in cellulose, hemicellulose and lignin structures. However, about 300 million tons of straw is incinerated or deserted in China every year, which is not only destruction of soil and atmospheric environment, but waste of resources. One of promising ways to utilize this precious bioresource is to produce straw-based adsorbents (Oei et al., 2009; Xu et al., 2011). Although the adsorption capacity of raw straw is unsatisfactory, various chemical modifications by introducing some functional groups can be applied to improve the adsorption capacity. In our previous work, wheat straw materials were modified by carboxymethylation to prepare a sort of anionic adsorbents, which were proved effective in eliminating cationic dyes through electrostatic interaction (Zhang et al., 2011). In this work, cationic modified straws were prepared by introducing quaternary ammonium groups through etherification. The cationic adsorbents were used to eliminate methyl orange (MO) and acid green 25 (AG25), two kinds of anionic dyes, from aqueous solutions. The fundamental adsorption behaviors of modified straw for removal of both MO and AG25, including the pH effect, adsorption isotherms, kinetics, and column adsorption, were investigated, respectively. Furthermore, after adsorption of dyes, regeneration and reuse of the used adsorbents is a problem. Conventionally, large amount of solvent is applied as eluent to wash and recover adsorbents for recycle use. However, the generated waste eluent usually requires further treatment to avoid the secondary pollution. Recently, a novel way to make use of used adsorbents has been raised in reported literatures. (Dai et al., 2011; Yan et al., 2011; Zhang et al., 2011) Since the surface structures of adsorbents have been changed after adsorption of some contaminant, the used adsorbents can be applied to adsorb other pollutants in suitable conditions. In this work, the final anionic dyes loaded adsorbents were tried to be removal of a cationic dye, methylene blue (MB). The adsorption mechanisms for various dyes onto different adsorbents were discussed in detail. 2. Methods

60–150 lm. The sieved straw was then washed with ethanol and distilled water. The washed materials were dried in an oven at 353 K for 24 h. The straw material used in the current study was named WS. A desired amount of WS was added in 30% NaOH solution under agitation at 293 K for 1 h to get the alkaline WS. After being washed and dried, the alkaline WS was dispersed in NaOH isopropanol-water solution. Then CTA was added dropwise into the mixture and left under agitation at 318 K for 3 h. The raw product was then washed by deionized water and dried at 353 K overnight. At last, it was ground and sifted to get the geometrical sizes (60–150 lm), named modified wheat straw (MWS), and used as the adsorbent in the following experiments. 2.3. Instrument analysis The FTIR spectra of WS and MWS were obtained with a Bruker IFS 66/S IR Spectrophotometer. All samples were prepared as potassium bromide tablets, and the scanning range was 650–4000 cm1. The surface morphologies of WS and MWS were observed directly with a scanning electron microscope (Type SSX-550; Shimadzu Co.; Japan). The electron micrographs were taken with an acceleration voltage of 15.0 kV. The f potential data was acquired from a Malvern Nano-Z f potential recorder. The range of initial solution pH was 2.0–12.0. The nitrogen content of MWS was obtained with an Elementar Vario EL II elemental analyzer. 2.4. Adsorption studies 2.4.1. Adsorption of anionic dyes at different initial solution pH The influences of different initial solution pH on adsorption of MO and AG25 onto MWS and WS were conducted, respectively. The initial concentrations of MO and AG25 solutions were both approximately 1000 mg L1. The initial pH value of MO solution was between 5.0 and 12.0, since MO was stable at pH higher than 5.0. The pH range of AG25 solution was between 2.0 and 12.0. 0.03 g adsorbent was immerged into 30 mL dye solutions with different pH under continuous stirring at 293 K for 48 h to achieve adsorption equilibrium. The initial and final pH values of these solutions were all detected by a pH meter (Type PHS-3C; Jinmai Corp.; China). The initial and final MO and AG25 concentrations were determined using Vis spectrophotometer (Type 7200; Unico Corp.; China) at fixed wavelengths of 464 and 642 nm, respectively, since the wavelength and intensity of the maximal absorbance for various dyes could not be affected by pH in the measured pH ranges as shown in the Supporting Information TEXT S1 and Fig. S2. The amount of adsorption in batch experiments, q (mg g1), was calculated according to the following equation:

2.1. Materials

q¼ The wheat straw used in the present study was obtained from a farm in Changzhou, Jiangsu Province. MO, AG25, MB, 3-chloro-2hydroxypropyl trimethylammonium chloride (CTA), hydrochloric acid (HCl), sodium hydrate (NaOH), ethanol, isopropanol 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 biosorbents The synthesis of cationic modified straw is summarized in Supporting Information Fig. S1. The straw materials were ground and screened through a set of sieves to obtain geometrical sizes of

41

ðC o  C e ÞV m

ð1Þ

where C0 and Ce (mg L1) are the initial and final concentrations of solute in solution, V (L) is the volume of the solution, and m (g) is the mass of the adsorbent. 2.4.2. Adsorption equilibrium study The adsorption equilibrium study was studied at different temperatures: 283, 293, 303, and 313 K, respectively. The concentration of MO aqueous solutions ranged from 100 to 2000 mg L1, whereas the concentration of AG25 aqueous solutions ranged from 150 to 3000 mg L1. 0.03 g MWS was dosed in each of the dye solutions under continuous stirring for 48 h at initial pH 7.0.

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W. Zhang et al. / Bioresource Technology 117 (2012) 40–47

2.4.3. Adsorption kinetics study The adsorption kinetics study was also conducted at varied temperatures: 283, 293, 303, and 313 K, respectively. The initial concentrations of MO and AG25 solutions were both 1000 mg L1. The initial solution pH was 7.0. 0.5 g MWS was immerged into 500 mL dye solutions under continuous stirring at varied temperatures. Then, 1.00 mL of sample solutions were taken out at desired time intervals to analyze the current dye concentration, meanwhile, 1.00 mL of water with pH 7.0 was added into the solutions to keep the volume constant. The adsorption capacity at time ti, q(ti) (mg g1), was calculated using the following equation:

ðC 0  C ti ÞV o  qðt i Þ ¼ m

Pi1 2

C ti1 V s

ð2Þ

2.5.2. Adsorption of MB at different initial solution pH The adsorption capacities of WS, MWS, MWS-MO and MWSAG25 for removal of MB were conducted by batch method at initial solution pH ranged from 2.0 to 10.0, adjusted by dilute H2SO4 or NaOH solutions, respectively. The initial concentration of MB solution was 500 mg g1. 0.03 g adsorbent was immerged into 30 mL MB solution for 48 h at 298 K. The final MB concentrations were detected at a wavelength of 662 nm by Vis spectrophoto meter.

3. Results and discussion 3.1. Characterization of the modified straw adsorbents

1

where C0 and Ct (mg L ) are the initial dye concentration and dye concentration at time ti, respectively. V0 and Vs (L) are the volume of the solution and that of the sample solution taken out each time for dye concentration analysis, respectively. And m (g) is the mass of the adsorbent. 2.4.4. Column experiments Column experiments were conducted using a glass column (internal diameter 12.8 mm, length 200 mm) equipped with water baths to keep the column temperature constant at 298 K. The mass of adsorbent was 1.0, 1.5, and 2.0 g, with corresponding bed depth of 3.8, 5.7, and 7.6 cm, respectively. In addition, the column adsorption of WS was also carried out for comparison. The concentrations of MO and AG25 solutions were approximately 2000 and 6000 mg L1, respectively. Initial pH was 7.0, and the flow rate was 1.28 mL min1. The amount of adsorption in column was calculated according to the equation given below:

qexp ¼

C0 v t  v m

Rt 0

C t dt

MWS has been prepared as described in the experimental part, and the FTIR spectra of WS and MWS were shown in Supporting Information Fig. S3. From Fig. S3, the broad peak around 3344 cm1 was the characteristic peak of WS corresponding to presence of free hydroxyl groups of cellulose and lignin. The strong C–O–C band at around 1024 cm1 also confirmed the cellulose and lignin structures. The weak band at 2902 cm1 was assigned to the stretch vibration of C–H bond in methylene groups. Compared to those of WS, the extra peak at 1478 cm1 in MWS verified the presence of C–N bond. It indicated that quaternary ammonium groups were successfully introduced to the chain backbone after modification. In addition, the amount of quaternary ammonium groups grafted onto the straw were roughly estimated around 1.74 mmol g1 from the nitrogen content obtained by an elemental analyzer. The SEM images of WS and MWS were shown in Supporting Information Fig. S4. The fiber structure of wheat straw was destroyed after modification. This was due to the fact that lignin

ð3Þ

where C0 (mg L1) is the influent dye concentration and Ct (mg L1) is the effluent concentration at time t (min), v (mL min1) is the flow rate of the effluent, m (g) is the mass of the adsorbent.

(a)

350

250

-1

2.5. Secondary adsorption of MB onto MO or AG25 loaded MWS

q (mg g )

300

2.5.1. Desorption studies The effects of different initial pH on desorption of MO and AG25 from MO and AG25 loaded MWS (MWS-MO and MWS-AG25) were studied at 298 K, respectively. MWS-MO and MWS-AG25 were weighed, and then immerged into varied aqueous solutions with different initial pH values ranged from 2.0 to 12.0 at 298 K for 48 h. The final dye concentrations in solution were analyzed using the same method as mentioned above to estimate the amount of desorption.

200

N

N N

150

SO3 Na

MO

100 50 0 4

5

6

7

8

9

10

11

12

13

pH

(b)1000 950

50

900 -1

q (mg g )

ς potential(mV)

40 30 20 10 0

850 800

Na SO3

80

o

o

N

N

SO3 Na

AG25

60 40

-10

20

-20

0

-30

2 4

6

8

10

12

4

6

8

10

12

pH

pH Fig. 1. The f potential of WS (h) and MWS (s) at varied pH conditions.

Fig. 2. The pH effects on MO (a) and AG25 (b) adsorption by WS (h) and MWS (s) at 298 K.

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W. Zhang et al. / Bioresource Technology 117 (2012) 40–47 Table 1 Adsorption capacities of MO and AG25 by several adsorbents. Dyes

Adsorbents

q (mg g1)

Reference

MO MO MO MO MO MO

MWS Activated alumina De-oiled soya Bottom ash Banana peel Orange peel

278.7–338.6 9.8 16.7 3.6 21 20.5

This study Iida et al. (2004) Mittal et al. (2007) Mittal et al. (2007) Annadurai et al. (2002) Annadurai et al. (2002)

MO AG25 AG25 AG25 AG25 AG25 AG25 AG25

Lapindo volcanic mud MWS Chitosan Date stones Durian peel Shells of bittim Sodium alginate/titania nanoparticle Modified starch

333.33 898.5–1015 645.1 36.90 63.29 16 151.5 1119

Jalil et al. (2010) This study Wong et al. (2004) Mahmoodi et al. (2010) Hameed and Hakimi (2008) Aydin and Baysal (2006) Mahmoodi et al. (2011) Wang et al. (2010)

and hemicellulose dissolved or degraded in the basification procedure, which would facilitate the etherification in the following step, thus enhance adsorption efficiency of final product. The pH dependence of f potential of WS and MWS was shown in Fig. 1. As in Fig. 1, the f potential of WS was minus in all pH range, indicating negative charge on the surface of WS. After modification, the f potential of MWS turned positive in all measured pH range, which confirmed further that quaternary ammonium groups were introduced to WS successfully.

However, as solution pH increased, quaternary ammonium groups were partially hydrolyzed, which resulted in the reduction of active sites on MWS as well as decrease of the dyes uptakes and f potential of MWS. In addition, the pH values of dye solutions before and after adsorption were also detected and shown in Supporting Information Fig. S5. The pH after adsorption of both MO and AG25 all decreased when the initial solution pH was higher than 7.0, which further supported the hydrolyzation effects of the quaternary ammonium groups by consumption of hydroxyl ions.

3.2. Adsorption studies 3.2.1. pH effect on adsorption of MO and AG25 The modified straw has been employed for adsorption of two anionic dyes from aqueous solutions. The pH effects have been investigated at the beginning since pH was one of the most important factors that might influence adsorption capacity. Fig. 2 showed pH dependence of WS and MWS for removal of MO and AG25, respectively. The MO and AG25 uptakes of WS were both kept at low level in all measured pH range. In contrast, the adsorption capacities of MWS for both dyes were much higher. The MO uptake increased from less than 20 mg g1 to more than 300 mg g1 after modification. Similarly, the AG25 uptake of MWS approached 1000 mg g1, which was over 20 times that of WS. From the results of nitrogen content as mentioned above, the adsorption efficiency of quaternary ammonium groups on MWS for both MO and AG25 could be estimated. It was found that nearly two quaternary ammonium groups could adsorb one MO ion, while approximately one quaternary ammonium group could adsorb one AG25 ion. This was probably due to the fact that AG25 had more negative charge than MO, thus had stronger electrostatic attraction with MWS. Besides, the adsorption capacity of MWS was also compared with some other adsorbents. As listed in Table 1, the adsorption capacities of several kinds of adsorbents for MO and AG25 have been reported in the previous literatures. It was found that the MO and AG25 uptakes of MWS were much higher than those of the mostly listed adsorbents, including chitosan, modified starch, activated palm ash and so on. In addition, with increase of initial solution pH, the adsorption capacities of MWS for MO and AG25 both slightly decreased. It was indicated that quaternary ammonium groups might play a very important role for adsorption of anionic dyes, since the structure of the quaternary ammonium group is sensitive to the external pH. As was known, anionic dyes had good affinity with cationic matters. After modification, MWS had abundant quaternary ammonium groups. The adsorptions of both MO and AG25 by MWS were mainly achieved by electrostatic interaction between anionic dye molecules and cationic quaternary ammonium groups.

3.2.2. Adsorption equilibrium study In order to describe the interactive behaviors between the solutes and adsorbents, the equilibrium study was conducted. The adsorption isotherms for MO and AG25 onto MWS at varied temperatures were presented in Supporting Information Fig. S6a and b, respectively. From Fig. S6, the adsorption isotherms showed no significant variations with temperature for both dyes. This phenomenon might indicate the absence of physical adsorption, which was usually notably affected by temperature. For further interpretation of adsorption behaviors, four common isothermal adsorption equilibrium models, Langmuir (Langmuir, 1918), Freundlich (Freundlich, 1907), Sips (Sips, 1948) and Dubinin–Radushkevich (D–R) models (Dubinin, 1947), were applied to fit the experimental data, respectively. The representation of adsorption equilibrium equations was shown in Supporting Information TEXT S2. The simulated parameters by aforementioned models were all listed in Table 2. It was found that thedetermination coefficients (R2) of the linear form for Langmuir model were much closer to 1.0 than those of other models for both dyes. Furthermore, from Table 2, the qL of MWS calculated from Langmuir model at varied temperatures were all close to their experimental adsorption capacities (qexp), respectively. It was indicated that Langmuir model was much better to describe the adsorption of both MO and AG25 onto MWS. From the viewpoint of adsorption mechanism in molecular level, MWS was abundant in quaternary ammonium groups after modification, and the anionic dyes were facile to be adsorbed onto MWS through electrostatic interaction, which was according with a monolayer chemical adsorption process assumed by Langmuir model. The fitting results of the Freundlich model were also listed in Table 2, in which the R2 of the Freundlich model were much lower than those of Langmuir model. The Freundlich model was applied to describe a heterogeneous system. In the present research, the lower R2 further supported the homogenous monolayer adsorption mechanism. Meanwhile, Sips model, which could be considered as a combination of Langmuir and Freundlich equations, was also applied. The

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W. Zhang et al. / Bioresource Technology 117 (2012) 40–47

Table 2 The isotherms parameters for MO and AG25 adsorption onto MWS. Dye

T (K)

qexp (mg g1)

Langmuir model 1

qL (mg g MO

AG25

283 293 303 313 283 293 303 313

338.6 326.7 308.5 278.7 963.3 1015 920.7 898.5

)

350.9 327.9 320.5 277.0 917.4 952.3 917.4 934.6

Freundlich model 1

bL (L g 33.99 33.38 32.69 25.12 54.50 50.60 48.38 40.26

)

R

2

0.9980 0.9988 0.9940 0.9966 0.9997 0.9987 0.9967 0.9967

KF

n

R

106.7 149.1 116.4 111.7 140.2 161.3 253.5 175.0

5.653 8.979 6.733 7.102 3.067 3.631 5.081 4.117

0.9011 0.9740 0.9184 0.8208 0.5687 0.5610 0.4262 0.4664

heterogeneity factor of n1 << 1 indicates heterogeneous adsorbents, while n1 close to or even 1.0 indicate materials with comparatively homogenous binding sites. In this case, the Sips model could be reduced to Langmuir equation. According to the analysis shown in Table 2, all of the calculated n1 were roughly close to 1.0, which indicated that MWS was a sort of homogenous adsorbent. Furthermore, based on the analysis from D–R model, the values of mean adsorption energy E would give information about adsorption mechanisms, physical or chemical process. The adsorption behaviors were ascribed to physical adsorption when E was between 1.0 and 8.0 kJ mol1, but chemical adsorption while E was higher than 8.0 kJ mol1. The calculated E for both MO and AG25 presented in Table 2 were all higher than 8.0 kJ mol1, although R2 was not very high. This further confirmed that the adsorption behaviors were chemical adsorptions. This phenomenon might result in the unobvious variations of the adsorption behaviors of MWS with temperature as mentioned above. Besides, adsorption equilibrium study at various temperatures could also provide valuable information of adsorption thermodynamics. The thermodynamic parameters for the adsorption process, namely DG (kJ mol1), DH (kJ mol1) and DS (kJ mol1 K1), could be calculated based on the given equations as shown below from Langmuir equilibrium constants.

DG ¼ RT ln K L

ð4Þ

DG ¼ DH  T DS

ð5Þ 1

Sips model 2

1

where R (J mol K ) is the gas constant, T (K) is the temperature, KL (dm3 mol1) is the Langmuir equilibrium constant, which can be obtained by multiplying the constant of b with the molar weight of dyes. The calculated DG for MO and AG25 adsorptions were both negative, indicating that the adsorption process was spontaneous. Furthermore, the calculated DH were 6.964 and 7.083 kJ mol1 for MO and AG25, while the DS were 53.31 and 61.90 J mol1 K1 for MO and AG25, respectively. It was proved that the dyes adsorption was a spontaneous and exothermal process driven by entropy increase.

qm (mg g

D–R model

1

)

380.1 407.3 354.2 296.4 900.5 921.4 895.5 818.9

b (L mg

1

0.0618 0.0485 0.1083 0.1705 0.0259 0.0460 0.1144 0.0622

)

2

n

R

1.9581 3.0974 2.4386 1.8112 0.3968 0.4334 0.4148 0.1482

0.9668 0.9949 0.9715 0.9669 0.9798 0.9677 0.8295 0.8988

E (kJ mol1)

R2

13.87 18.57 14.52 20.67 9.164 10.61 15.14 12.11

0.7335 0.7818 0.7426 0.5867 0.6680 0.6565 0.4816 0.5379

3.2.3. Adsorption kinetics The adsorption kinetics has been also investigated. The experimental results of MO and AG25 adsorption on MWS versus time at various temperatures were shown in Fig. S7. As in Fig. S7, the adsorption equilibrium for MO was quickly achieved within 20 min, indicating a very fast adsorption rate. The adsorption was a little slower for AG25, but the adsorption equilibrium was still achieved within 40 min. Meanwhile, the adsorption kinetics also showed no significant variation with temperature. In order to investigate the adsorption mechanisms further, four popular kinetic models were applied to fit the experimental data, which were pseudo-first order (Lagergren, 1898), pseudo-second order (Ho and McKay, 1998), Elovich (Low, 1960), and intraparticle diffusion model (Yahaya et al., 2009), respectively. The representation of kinetics equations was shown in Supporting Information TEXT S3 and the fitted parameters were all listed in Table 3. From R2 of various kinetics models, pseudo-second order model was more suitable to describe the adsorption kinetics behaviors than pseudo-first order model. It indicated that the rate controlling mechanism for adsorption was chemisorption, which was in accordance with those drawn from adsorption isotherm analysis. Meanwhile, the values of R2 fitted by intraparticle diffusion model were not very high, indicating that intraparticle diffusion was not the rate-limiting step in the adsorption process. Besides, Elovich equation also fitted the experimental data well, especially for AG25. As Elovich model was commonly suitable to describe the adsorption kinetics of ion exchange process, it was revealed that the adsorption behaviors of anionic dyes on MWS were not only chemisorptions, but ion exchange reactions. The adsorption mechanism was summarized in Fig. 3, which described the dyes adsorption process. This mechanism was quite similar to MB adsorption by carboxymethyl straw in our previous work (Zhang et al., 2011). Furthermore, on the basis of the rate constants of the pseudosecond order kinetics at various temperatures, the activation energy Ea (J mol1) could be estimated according to the Arrhenius equation:

Table 3 The kinetics parameters for MO and AG25 adsorption onto MWS. Dye

T (K)

qexp (mg g1)

Pseudo-first order model

Pseudo-second order model

Elovich model

k1 (s1)

R2

k2 (103min1)

R2

AE

MO

283 293 303 313

271.2 293.1 294.4 287.3

0.2975 0.1931 0.3139 0.3128

0.8879 0.8463 0.6256 0.7280

5.337 8.066 8.684 9.264

0.9998 0.9996 0.9999 0.9993

259.4 253.1 232.9 206.0

AG25

283 293 303 313

934.3 890.5 957.1 894.3

0.08070 0.03508 0.03488 0.03641

0.9598 0.9748 0.8470 0.9090

0.7875 0.8758 0.8896 0.9067

0.9999 0.9936 0.9982 0.9944

530.5 399.6 489.2 351.1

BE 10.03 12.55 9.330 29.26 121.9 105.8 109.7 113.0

Intraparticle diffusion model R2

kp (mg g1 min0.5)

R2

0.9799 0.9428 0.9072 0.9968

100.4 56.92 4.710 8.511

0.7939 0.5998 0.8541 0.8632

0.9884 0.9870 0.9679 0.9541

74.08 58.70 60.58 71.59

0.7772 0.9573 0.8146 0.8757

45

W. Zhang et al. / Bioresource Technology 117 (2012) 40–47

N N

(a) N

N

N CH2N (CH3)3Cl

N

CH2N (CH3)3 CH2N (CH3)3 N SO3

CH2N (CH3)3Cl

CH2N (CH3)3 N

N N

S

N

SO3 Na

CH2N (CH3)3

N

SO3

N S

N CH2N (CH3)3Cl

ion exchange

electrostatic adsorption

CH2N (CH3)3 SO3

CH2N (CH3)3Cl

N

CH2N (CH3)3 SO3

CH2N (CH3)3 CH2N (CH3)3

N

N

N

N

N

N

(b)

CH2N (CH3)3 Cl

Na SO3

o

o

N

N

SO3

SO3 Na

o

o

N

N

SO3

N

CH2N (CH3)3

SO3

o

o

N

N

SO3

CH2N (CH3)3 N

N

N

electrostatic adsorption

ion exchange

CH2N (CH3)3 Cl

S

N

S

N

CH2N (CH3)3 CH2N (CH3)3 N

N

o

o

SO3

SO3

N

N

o

o

SO3

SO3

Fig. 3. Adsorption mechanisms of MO onto MWS & MB onto MWS-MO (a) and AG25 onto MWS & MB onto MWS-AG25 (b).

k ¼ AA exp

  Ea RT

ð6Þ

where AA is the Arrhenius constant. Ea could provide valuable information on the mechanism of the ion exchange process. In general, the adsorption process was classified to be film-diffusion controlled when Ea was below 16 kJ mol1, particle-diffusion controlled when Ea was 16–40 kJ mol1, and chemical-reaction controlled when Ea was greater than 40 kJ mol1. (Boyd and Soldano, 1953) The Ea for MO and AG25 adsorption were 3.934 and 15.51 kJ mol1, respectively. It suggested that the adsorption of the anionic dyes was a film-diffusion-controlled process.

through time for MO and AG25 both increased with increasing bed height as expected. It was due to the fact that MO and AG25 had more time to contact with MWS as the bed height increased, resulting in longer exhausting time and higher removal amount. Furthermore, the column adsorption capacity and the adsorption rate constant have been calculated using Thomas model (Thomas, 1944). Thomas model is one of the most popular models in column adsorption study, which assumes plug flow behavior in fixed-bed, and uses Langmuir isotherm for equilibrium, and second-order reversible reaction kinetics. The expression of Thomas model is given as follows:

3.3. Column study

Ct 1 ¼ C 0 1 þ expðkTh ðqTh m  C 0 v tÞÞ v

In real applications, the column technique is one of the most common ones. In this study, the column experiment of MWS for both MO and AG25 were conducted and the typical breakthrough curves were plotted in Supporting Information Fig. S8. The break-

where kTh (mL min1 mg1) is Thomas rate constant, qTh (mg g1) is the theoretical saturate adsorption capacity in Thomas model, v (mL min1) is the flow rate of the effluent, m (g) is the mass of the adsorbent, C0 (mg L1) is the influent dye concentra-

ð7Þ

Table 4 Thomas model parameters for adsorption of MO and AG25 in fixed-bed systems at 298 K. Dyes

Adsorbents

Z (cm)

m (g)

kTh (mL mg1 min1)

qexp (mg g1)

qTh (mg g1)

R2

AG25

WS MWS

6.8 3.8 5.7 7.6

1.5 1.0 1.5 2.0

0.0547 0.0111 0.00839 0.00395

60.0 860.4 873.4 980.0

54.8 811.0 843.9 974.6

0.9296 0.9862 0.9775 0.9938

MO

WS MWS

6.8 3.8 5.7 7.6

1.5 1.0 1.5 2.0

0.169 0.0164 0.0117 0.00732

38.2 287.1 312.1 324.9

42.5 320.2 345.4 378.4

0.9933 0.9899 0.9949 0.9932

W. Zhang et al. / Bioresource Technology 117 (2012) 40–47

3.4. Secondary adsorption of MB onto MWS-MO and MWS-AG25

(a)

Percentage of desorbed MO(%)

As was known, the reusability was a very important factor for developing a novel adsorbent in practice applications. As mentioned above, the traditional treatment for these used adsorbents was to wash and recover for recycle use. Recently, a novel way to make use of these used adsorbents has been reported in the literatures (Dai et al., 2011; Yan et al., 2011; Zhang et al., 2011). Since the surface structures of adsorbents have been changed after covered by a layer of contaminant, the used adsorbents can be applied to adsorb other pollutants at suitable conditions. In this work, the anionic dyes loaded straws were tried to remove a cationic dye, MB. Before this treatment, the stability of MWS-MO and MWS-

100 80 60 40 20 0 2

4

6

8

10

12

(b)

Percentage of desorbed AG25(%)

pH 100

140 120 100 -1

tion, Ct (mg L1) is the effluent concentration at time t (min). The kinetic coefficient kTh and the adsorption capacity of the column qTh can be determined from a plot of Ct/C0 against t at a given flow rate using non-linear regression. The fitted curves were also shown in Supporting Information Fig. S8, and their parameters were all listed in Table 4. It was found that the theoretic curves fitted the experimental data in a good way, and R2 at varied bed heights were all close to 1.0. Furthermore, the theoretic dye uptakes qTh at varied bed heights were very close to their respective experimental ones. It was indicated that Thomas model was suitable to describe the adsorption of both MO and AG25 in fixed-bed systems. In addition, the rate constant kTh decreased with the bed height increased, which was ascribed to the fact that higher bed depth would result in longer contact time and decrease of reaction rate. Since Thomas model was based on Langmuir model and pseudo-second order reversible reaction kinetics (Thomas, 1944), the well fitness of this model further confirmed that the adsorptions of MO and AG25 by MWS were monolayer chemical adsorption, which was fully consistent with the analyses in the batch studies.

q (mg g )

46

80 60 40 20 0 2

4

6

8

10

pH Fig. 5. The pH effects on MB adsorption by WS (h), MWS (s), MWS-MO (4), and MWS-AG25 (5) at 298 K, respectively.

AG25 at various pH conditions was studied firstly, and the results were shown in Fig. 4. It was found that, at pH 12.0, MO and AG25 would be partially desorbed from MWS, owing to excessive hydroxyl ions competing the reaction sites on the adsorbent and substituting anionic dyes. However, when pH was lower than 10.0, there was little desorption, and both MWS-MO and MWS-AG25 were stable enough for further applications. Consequently, the secondary adsorptions of MB onto the used adsorbents were carried out at pH lower than 10.0. From Fig. 5, MB uptake of MWS-MO was between 100 and 130 mg g1 while that of MWS-AG25 was between 80 and 110 mg g1. In addition, the adsorption of MB by the used adsorbents did not change notably with pH in the measured pH range. After adsorption of MO and AG25, the surfaces of MWS-MO and MWS-AG25 were covered with a layer of anionic dyes, which could adsorb cationic MB through electrostatic interaction. In contrast, the adsorption capacity of MWS was less than 20 mg g1, owing to electrostatic repulsion between MB and quaternary ammonium groups. Meanwhile, the MB uptake of WS increased with solution pH and reached around 50 mg g1 at higher pH range, resulting from negative charge on the surface of WS as mentioned above, but it was still much less than those of MWS-MO and MWS-AG25. Furthermore, based on the adsorption capacities, the molar amounts of MO (0.92 mmol g1) and AG25 (1.50 mmol g1) loaded on MWS were both roughly 2.0 times that of MB adsorbed in the secondary adsorption (0.40 mmol g1 on MWS-MO and 0.66 mmol g1 on MWS-AG25). It was concluded that the adsorption capacity of MB was in connection with the amount of anionic dyes loaded on MWS. From the quantitative relationship of their adsorption capacities, it was suggested that one MB ion might be electrostaticly adsorbed by approximately two anionic dye ions, since there was no sufficient negative charge and/or space on the adsorbents’ surface. The secondary adsorption mechanisms were also described in Fig. 3.

80

4. Conclusion 60 40 20 0 2

4

6

8

10

12

pH Fig. 4. The pH effects on MO desorption from MWS-MO (a) and AG25 desorption from MWS-AG25 (b) at 298 K.

Above all, the straw materials after proper modification were effective in removal of both MO and AG25 from aqueous solutions. The adsorptions of the anionic dyes showed pH dependent, but no significant variations with temperature. The further adsorption mechanism studies indicated that it was a monolayer chemical adsorption and the rate-limiting step might be ion exchange reaction, which was also a spontaneous and exothermal process. In addition, the used adsorbents were employed to adsorb MB from aqueous solutions directly and showed high removal efficiency at suitable conditions, indicating that the secondary adsorption was an efficient and economical way for reuse of the used adsorbents.

W. Zhang et al. / Bioresource Technology 117 (2012) 40–47

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