Simultaneous removal of acid green 25 and mercury ions from aqueous solutions using glutamine modified chitosan magnetic composite microspheres

Environmental Pollution 209 (2016) 21e29

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Simultaneous removal of acid green 25 and mercury ions from aqueous solutions using glutamine modified chitosan magnetic composite microspheres Xue Tao, Kun Li, Han Yan, Hu Yang*, Aimin Li State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 September 2015 Received in revised form 12 November 2015 Accepted 14 November 2015 Available online xxx

In this current work, the magnetic composite microsphere containing glutamine modified chitosan and silica coated Fe3O4 nanoparticles (CS-Gln-MCM) has been successfully prepared and extensively characterized, which is a kind of biodegradable materials. CS-Gln-MCM shows enhanced removal efficiency for both acid green 25 (AG25), an amphoteric dye, and mercury ions (Hg2þ) from water in the respective while measured pH range compared with chitosan magnetic composite microsphere (CS-MCM) without modification. It is due to the fact that the grafted amino acid provides a variety of additional adsorption active sites and diverse adsorption mechanisms are involved. In AG25 and Hg2þ aqueous mixture, the modified adsorbents bear preferential adsorption for AG25 over Hg2þ in strong acidic solutions ascribed to multiple interactions between AG25 and CS-Gln-MCM, such as hydrogen bonding and electrostatic interactions. While, in weak acidic conditions, an efficient simultaneous removal is observed for different adsorption effects involved in aforementioned two pollutants. Besides, CS-Gln-MCM illuminates not only short equilibrium time for adsorption of each pollutant less than 20.0 min but also rapid magnetic separation from water and efficient regeneration after saturated adsorption. Therefore, CS-Gln-MCM bears great application potentials in water treatment. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Glutamine modified chitosan magnetic composite microspheres Acid green 25 Hg2þ Simultaneous removal Adsorption mechanism

1. Introduction China is a major textile manufacturer in the world, and large quantities of dyestuffs have been produced and consumed every year (Xin, 2006). However, the generated dyeing effluents in the textile industry result in a serious environmental concern due to their strong toxicity, increased chemical oxygen demand (COD), uneasy biodegradation, high light absorbance, serious retardation of photosynthesis, and inhibition of normal growth of aquatic organisms (Anliker, 1979). Therefore, a great many of technologies have been employed for the removal of dyes from wastewater (Robinson et al., 2001). They include adsorption (Chiou and Chuang, 2006; Gupta and Suhas, 2009; Ngah et al., 2011), chemical precipitation (Vimonses et al., 2010), electrolytic chemical treatment (Martinez-Huitle and Brillas, 2009), chemical reduction/oxidation (Monteiro Paschoal et al., 2009; Nidheesh et al., 2013), membrane technology (Dasgupta et al., 2015; Doke and Yadav, 2014) and

* Corresponding author. E-mail address: [email protected] (H. Yang). http://dx.doi.org/10.1016/j.envpol.2015.11.020 0269-7491/© 2015 Elsevier Ltd. All rights reserved.

biological methods (Banat et al., 1996; Saroj et al., 2014; Tastan et al., 2010). Comparatively, adsorption has gained great advantages in reducing or minimizing various kinds of dyes in wastewater for its highly efficient and cost-effective characteristics especially in advanced treatment. However, many kinds of dyeing and finishing auxiliaries, such as scouring auxiliaries, fixing, leveling, and dispersing agents, have been also used at the same time to improve the dyeing performance in the textile industry (Chen, 2010). So those discharged dye effluents are increasingly complicated for still containing a number of dyestuff assisters besides dyes resulting in a great challenge and difficulty for efficient treatment (Fu et al., 2013; Vimonses et al., 2010). Among them, various heavy metal ions, such as chromium, mercury, and copper, are usually involved (Correia et al., 1994; Li and Fu, 2014). Those heavy metal ions in wastewater have also posed serious threat to the environment and human being (Jarup, 2003). For example, mercury is one of the most toxic heavy metals, which maximum uptake by human is no more than 0.3 mg per week recommended by the World Health Organization (WHO, 2011). Therefore, it is a significant issue for efficient removal of both dyes and heavy metal ions from wastewater (Al-Ghouti et al., 2010;

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X. Tao et al. / Environmental Pollution 209 (2016) 21e29

Reddy and Lee, 2013). As mentioned above, adsorption is one of suitable techniques for the treatment of dyeing effluents, which is also effective to heavy metal ions (Ngah et al., 2011). However, most of current reported works have made close study of single pollutant system, and few has focused on simultaneous removal of both dyes and heavy metal ions from their mixtures by adsorption (Deng et al., 2013; Wang et al., 2015). As is known, choosing the right and appropriate adsorbents for the targeted pollutants should be given top priority for adsorption technique (Dabrowski, 2001). In recent years, researchers have taken considerable interests in development of novel and high-efficient adsorbents derived from natural polymers such as chitosan (Ngah et al., 2011; Rinaudo, 2006), starch (Crini, 2005), and cellulose (Chmielewska, 2008; Crini, 2005) due to their abundance in nature as well as nontoxic, biodegradable, environment-friendly, and low-cost advantages. Chitosan, one of high-performance polysaccharide materials, has been applied as an efficient adsorbent for the removal of various pollutants from water, since it contains abundant amino and hydroxyl groups on the chain backbone (Crini and Badot, 2008; Ngah et al., 2011; Pillai et al., 2009; Rinaudo, 2006; Thakur and Thakur, 2014). Moreover, it is a significant strategy to give some chemical modifications like etherification, grafting, or cross-linking reactions for further enduing chitosan with superior adsorption properties on the basis of the characteristics of target pollutants and structure activity relationship (Pillai et al., 2009; Rinaudo, 2006). Some special and extra functional groups have been introduced onto chitosan which greatly improve its adsorption affinity towards the contaminants (Crini and Badot, 2008). In this current work, acid green 25 (AG25) and mercury ions (Hg2þ) have been selected as targeted pollutants, since AG25 is a kind of popular amphoteric dyes and Hg2þ as a component of dyeing and finishing auxiliaries usually exists in dyeing effluents (Elsasser, 2005). From aforementioned modification strategy, glutamine, a kind of amino acids containing amino, carboxyl, and amide groups, has been chosen as the modifying agent to enhance the adsorption capacity of chitosan for AG25 and Hg2þ. Moreover, it is practically significant that a used adsorbent could be separated from water quickly. In recent years, magnetic separation technology was developed and employed to accelerate the separation process for adsorbents (Ambashta and Sillanpaa, 2010). In this work, magnetic Fe3O4 nanoparticles were coated by a layer of silica to improve their stabilities in strong acidic conditions, then which were embedded into chitosan matrix, forming a coreeshell structure (Wang et al., 2010). Thus glutamine modified chitosan magnetic composite microspheres (CS-Gln-MCM) have been obtained. The simultaneous removal of AG25 and Hg2þ from their mixtures by CS-Gln-MCM has been studied in addition to its fundamental adsorption behavior such as initial solution pH dependence, adsorption isothermal equilibrium and kinetics in respective single pollutant system. Moreover, the recycling and reuse of the used adsorbents were also performed due to its great importance in practical applications. 2. Experiments 2.1. Materials Chitosan used in this study was purchased from Shandong Aokang Biological Co. Ltd. Its deacetylation degree and viscosityaverage molecular weight are 90.5% and 1.0  106 g mol1 respectively. Ammonium iron sulfate hexahydrate ((NH4)2Fe(SO4)2$6H2O), iron chloride hexahydrate (FeCl3$6H2O), glutaraldehyde (GLA) solution (25%, w/w), cyclohexane and Na2SiO3$9H2O were also obtained by Sinopharm Chemical Reagent Co. Ltd. Span

80 (Sorbitan Monooleate) was provided by Shenyu Chemical Reagent Co. Ltd. Acid Green 25 was purchased from Tianjin Institute of Chemical Reagents. Hg(NO3)2, glutamine, hydrochloric acid (HCl), sodium hydrate (NaOH), acetic acid, and other reagents used in this work were from Nanjing Chemical Reagent Co. Ltd.. All the reagents are of analytical grade, and distilled water were used in all experiments. 2.2. Preparation of magnetic adsorbents 2.2.1. Preparation of silica-coated Fe3O4 magnetic nanoparticles The co-precipitation method was applied to prepare magnetic Fe3O4 nanoparticles. The mole ratio of Fe2þ and Fe3þ salts was kept constant at 2:1. They were first dissolved in HCl aqueous solution under N2 atmosphere to avoid oxidation of Fe2þ and then added dropwise into NaOH aqueous solution under vigorous stirring at 353 K and N2 atmosphere for 1.0 h. Finally, Fe3O4 nanoparticles were obtained after filtering and washing with distilled water for several times. Furthermore, the obtained magnetic nanoparticles were coated by a layer of silica to improve its acid resistance (Wang et al., 2010). In detail, an appropriate amount of Na2SiO3 aqueous solution was added dropwise into the Fe3O4 suspensions by vigorous stirring at 353 K under N2 atmosphere. The solution pH was adjusted to 6.0 by dilute HCl or NaOH aqueous solutions. After reaction for 1.0 h, the silica coated nanoparticles (Fe3O4@SiO2) were generated, then washed to neutral condition using distilled water for several times. At last, the resultant Fe3O4@SiO2 was kept in water for further use. 2.2.2. Preparation of CS-Gln-MCM 1.6 g of chitosan powder and 2.18 g of glutamine were first fully dissolved in 100.0 cm3 of Fe3O4@SiO2 suspension. The mixture was then added slowly into 500.0 cm3 of cyclohexane which contained a small amount of Span 80 under vigorous stirring at 298 K. After the aforementioned suspension has been mixed evenly, 5 cm3 of glutaraldehyde solution (25%, w/w) was dropwise added into the water-cyclohexane emulsion as the crosslinker. After one hour's reaction at 323 K, the obtained product was filtered and washed using ethanol and distilled water in succession. Finally, it was kept in water for next adsorption study. In addition, CS-MCM was also prepared using the similar method to CS-Gln-MCM as control sample, except that glutamine was replaced by acetic acid during the first preparation step. The molecular structure and surface morphology of these newly synthetic materials have been characterized by fourier transform infrared spectrometer (FTIR), x-ray diffraction spectrometer (XRD), vibrating-sample magnetometer (VSM), scanning electron microscope (SEM), and thermos-gravimetric analyzer (TG). The operation details of those apparatus are all described in Supporting Information Table S1. 2.3. Adsorption of AG25 and Hg2þ in respective single system 2.3.1. Initial solution pH dependence The effects of initial solution pH on adsorption of AG25 and Hg2þ by CS-MCM and CS-Gln-MCM were conducted at 298 K. The range of initial solution pH was 2.0e10.0 for AG25 but 2.0 to 6.0 for Hg2þ in case of the precipitation formation when pH higher than respective upper limit. The adjustment of the initial solution pH was conducted using diluted HCl or NaOH aqueous solutions. Then, 0.03 g of dried adsorbent was dispersed in 30.0 cm3 of AG25 or Hg2þ aqueous solution with desired initial pH under continuous stirring for 2.0 h to achieve adsorption equilibrium. The initial concentrations of AG25 and Hg2þ were 800 mg dm3 and 200 mg dm3 respectively, at which AG25 and Hg2þ could both

X. Tao et al. / Environmental Pollution 209 (2016) 21e29

reach to the saturated adsorption. The final concentration of AG25 was analyzed using the Victor 722 Vis Spectrometer at the detective wave length of 642 nm. While that of Hg2þ was determined on a Thermo M6 atomic absorption spectrometer. All the results were measured three times, and the final concentration results were the average of three runs. Equilibrium adsorption capacity (qe, mg g1) was a fundamental factor to describe adsorbents' ability for removal of targeted pollutants. Here, it was calculated according to the following equation:

qe ¼

ðC0  Ce ÞV m

(1)

where C0 and Ce (mg dm3) are the initial and adsorption equilibrium concentrations of AG25 or Hg2þ here; V (dm3) is the volume of solution; m (g) is the dried weight of adsorbent. 2.3.2. Adsorption equilibrium study The adsorption equilibrium experiments were conducted at 298 K with pH 7.0 for AG25 and pH 5.0 for Hg2þ under different initial pollutant concentrations. The ranges of initial concentrations of AG25 and Hg2þ were from 0 to 1100 mg dm3 and 0e400 mg dm3 respectively. A 0.03 g of dried adsorbent was completely dispersed in 30.0 cm3 pollutant solution under continuous stirring for two hours to reach adsorption equilibrium. The final concentrations were tested as mentioned in Section 2.3.1. The adsorption capacities were estimated based on Eq. (1). 2.3.3. Adsorption kinetics study The adsorption kinetics experiments were carried out under the same conditions as adsorption equilibrium. The initial concentrations of AG25 and Hg2þ were 800 mg dm3 and 200 mg dm3 respectively. A 0.3 g of dried adsorbent was added into 300.0 cm3 pollutant solution under continuous stirring. Then, at the desired intervals, 0.1 cm3 of the mixed solution was taken out to analyze current concentration while adding the same volume of pure water into the bulk solution to keep the total volume constant. The adsorption uptake q(ti) (mg g1) at time ti was calculated using the following equation,

qðti Þ ¼

P ðC0  Cti ÞVo  i1 2 Cti1 Vs m

(2)

where C0 and Cti (mg dm3) are the initial concentration and concentration at time ti of pollutant respectively. V0 and Vs (dm3) are the volume of the mixed solution and that of the sample solution taken out each time for concentration analysis, respectively. Here, Vs is equal to 1.0 cm3. Finally, m (g) is the dried weight of the adsorbent. 2.4. Simultaneous adsorption study in AG25 and Hg2þ binary system The simultaneous adsorption experiments were carried out in AG25 and Hg2þ binary aqueous solution system at 298 K under various initial solution pHs. The initial concentrations of AG25 and Hg2þ were both kept at 1.5 mmol dm3. And the range of initial solution pH was 2.0e5.5. The other experimental conditions were similar to those in single pollutant system as described in detail in Section 2.3.1. 2.5. Desorption and reusability experiments Desorption behavior of CS-Gln-MCM after saturated adsorption was investigated at 298 K for recycling and reusing the used

23

adsorbents. After efficiently separated from the aqueous solutions under an external magnetic field, AG25-and Hg2þ-loaded CS-GlnMCMs were then regenerated by 0.1 mol dm3 NaOH and 0.01 mol dm3 EDTA aqueous solution respectively under continuous stirring for 24.0 h. The regenerated adsorbents were applied to next adsorption cycle for five times totally at room temperature. The adsorption experiments were conducted at pH 7.0 for AG25 and pH 5.0 for Hg2þ, and the initial concentrations of AG25 and Hg2þ for adsorption were 800 mg dm3 and 200 mg dm3 respectively. The adsorption capacities in each cycle were calculated on the basis of Eq. (1). 3. Results and discussion 3.1. Characteristics of the adsorbents Two kinds of chitosan-based magnetic adsorbents have been successfully obtained by only one-step method which was briefly described in experimental part and Scheme 1. Glutaraldehyde is used as a crosslinker here. The molecular structures of CS-MCM and CS-Gln-MCM have been investigated using various characterization methods, such as FTIR, XRD, VSM, TG, and SEM. Fig. 1 show FTIR spectra of two chitosan-based adsorbents before and after adsorption of various pollutants. From Fig. 1a and b, the characteristic peaks of chitosan have been observed in both CS-MCM and CS-Gln-MCM, i.e., the broad peaks about 3100e3400 cm1 due to eO-H and eNeH stretching vibrations, the peak at near 2900 cm1 ascribed to eCeH stretching vibration, and the overlapped peak at 1028 cm1 assigned to eCeN and eCeOH stretching vibrations, respectively (Dabrowski, 2001; Rinaudo, 2006). In particular, the overlapped characteristic peak at 1640 cm1 corresponds to eNeH stretching and eC]N stretching in spectrum of CS-MCM, indicating that chitosan has been chemically crosslinked using glutaraldehyde by amine aldehyde reaction (Hoffman, 2004) successfully. Moreover, the characteristic peaks at 1560 cm1 and 1377 cm1 both ascribe to eCOOe groups in spectrum of CS-Gln-MCM (Dai et al., 2010), which confirms that amino acid has been introduced onto the microspheres. Here, glutaraldehyde plays not only the crosslinker but also connecting reagent. The available detailed molecular structure of CS-Gln-MCM is also shown in Scheme 1. XRD patterns of Fe3O4, CS-MCM, and CS-Gln-MCM are presented in Supporting Information Fig. S1. Characteristic peaks at 2q ¼ 30.3 , 35.7, 43.4 , 53.92 , 57.28 , and 62.88 are ascribed to the cubic spinel structure of Fe3O4 crystals (Wang et al., 2010) which are displayed in all three measured samples, indicating that the magnetic nanoparticles have been well embedded in polymer matrixes. The VSM curves of the chitosan-based magnetic adsorbents are showed in Fig. 2. The similar magnetic hysteresis loops of CS-MCM and CS-Gln-MCM with saturation magnetizations of approximately 5.05 and 3.48 emu g1 respectively have been observed. However, an evident reduction is observed in comparison to that of pure Fe3O4 nanoparticles reported in authors' previous work (Yan et al., 2012), since the diamagnetic shell of the coated SiO2 and organic matters greatly reduce the contents of Fe3O4 nanoparticles in whole adsorbents (Yan et al., 2012). Still, it is enough to guarantee an effective magnetic separation of the two chitosan-based microspheres from aqueous solutions under external magnetic fields. From the insets, CS-Gln-MCM could be separated very rapidly under an external magnetic field. The results of thermal stability of the magnetic adsorbents are showed in Supporting Information Figure S2. It suggests that the organic matters in both microspheres would be fully thermodecomposed before 700  C. According to the TG analysis, the total

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X. Tao et al. / Environmental Pollution 209 (2016) 21e29

Scheme 1. Chemical crosslinking reaction of chitosan modified with glutamine and brief description for available adsorption mechanism of CS-Gln-MCM for removal of AG25 and Hg2þ.

content of organic matters in CS-MCM and CS-Gln-MCM is about 83% and 70% respectively. It is due to the fact that glutamine has grafted onto the chitosan since CS-Gln-MCM and CS-MCM have the same feeding amounts of chitosan and Fe3O4@SiO2 in their

f e d

Magnetization (emu.g )

6 -1

c b a

4000

preparation processes. Moreover, the amount of SiO2 modified Fe3O4 particles in the magnetic composite microspheres is also estimated, 17% for CS-MCM and 30% for CS-Gln-MCM, since Fe3O4@SiO2 could be still stable enough in current TG measured temperature range. It may result in higher saturation magnetization of CS-MCM shown in Fig. 2 as mentioned above. The surface morphologies of the two magnetic microspheres observed by SEM are displayed in Supporting Information Fig. S3. It

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 1. FTIR spectra of CS-MCM (a), CS-Gln-MCM (b), CS-MCM after adsorption of AG25 (c), CS-Gln-MCM after adsorption of AG25 (d), CS-MCM after adsorption of Hg2þ (e), and CS-Gln-MCM after adsorption of Hg2þ (f).

a

4

b

2 0 -2 -4 -6

-100

-50

0

50

100

Magnetic Field (KOe) Fig. 2. Magnetic hysteresis loops of CS-MCM (a) and CS-Gln-MCM (b). The insets are the separation effects of CS-Gln-MCM before and after using magnetic field.

X. Tao et al. / Environmental Pollution 209 (2016) 21e29

is found that CS-MCM and CS-Gln-MCM are both spherical particles but with rougher surfaces. The diameter of CS-Gln-MCM (40e45 mm) is smaller than that of CS-MCM (60e65 mm), indicating a larger surface area of CS-Gln-MCM and hence a better adsorption performance expected.

3.2. Removal of AG25 and Hg2þ in their respective singlecomponent system 3.2.1. Initial solution pH dependence The prepared chitosan-based magnetic microspheres are employed as adsorbents to remove AG25 and Hg2þ from respective aqueous solution. The results are shown in Fig. 3. From Fig. 3, the adsorption capacities of CS-Gln-MCM are significantly enhanced in comparison to those of CS-MCM in the respective whole measured pH range. These improvements could be attributed to the grafted glutamine, in which additional amino, carboxyl and amide groups are introduced. However, the adsorption capacities of both chitosan-based adsorbents for AG25 and Hg2þ have fully opposite pH dependences. The AG25 uptakes of CS-MCM and CS-Gln-MCM gradually decrease as pH increased. Strong acidic condition is favorable for the adsorption of AG25, as the surface of the adsorbents turn more positive for formation of much more eNHþ 3 groups on the chitosan backbone, which have higher affinity to the anionic sulfate groups of AG25. Furthermore, the secondary amine groups of dye and

Fig. 3. Effects of initial solution pH on AG25 (B/C, in a) and Hg2þ (,/-, in b) uptakes by CS-MCM/CS-Gln-MCM in respective single pollutant system.

25

carboxyl groups on glutamine could easily form hydrogen bonding (Crini and Badot, 2008; Kyzas et al., 2015; Ngah et al., 2011) as briefly described in Scheme 1. From Fig. 1c and d, the wavenumber and sharp of the characteristic peaks for eNH2 at 1028 cm1 and 1640 cm1 as well as those for carboxyl groups at 1560 cm1 and 1377 cm1 are shifted and splitted in the FTIR spectra of CS-MCM and CS-Gln-MCM after AG25 adsorption. It is further confirmed that these dyes have effectively bound to the chitosan-based adsorbents by some efficient interactions such as electrostatic interaction and hydrogen bonding. However, increasing pH would result in deprotonation and reduction of the positive surface charge, thus weakening their affinity to AG25. Still CS-Gln-MCM retains relatively high AG25 removal efficiency, as pH value further increases even at strong alkaline conditions. This could be attributed to the interactions between the secondary amine groups of dye and carboxyl groups on glutamine (Crini and Badot, 2008; Kyzas et al., 2015; Ngah et al., 2011) as mentioned above. On the other hand, the adsorption capacities of CS-MCM at higher pH levels reduce to near zero for lack of effective interactions between adsorbate and adsorbent. As for the pH dependence of Hg2þ uptakes of aforementioned two magnetic adsorbents, it shows a different trend to that of AG25 from Fig. 3. Hg2þ uptakes linearly increase from pH 2.0 to 5.0 and then gradually reach to the platform. As for CS-Gln-MCM, besides the chelating effects between the Hg2þ and the amino or hydroxyl groups of chitosan (Miretzky and Cirelli, 2009; Ngah et al., 2011; Rinaudo, 2006), the adsorption of Hg2þ is still attributed to the additional carboxyl and amide groups from glutamine resulting in its higher adsorption efficiency (Li et al., 2015; Miretzky and Cirelli, 2009). Moreover, from Fig. 1e and f, the overlapped characteristic peak of eNH2 and eOH at 1028 cm1 (Dabrowski, 2001) has no evident change in the FTIR spectra of two chitosan-based adsorbents after Hg2þ adsorption, but the characteristic peaks of eC]O at 1740 cm1 and 1377 cm1 turn broad indicating that Hg2þ has some strong interactions with carbonyl groups. However, the heavy reduction of Hg2þ uptakes at lower pH levels is due to the fact that more protons would stronger compete with the metal ions for aforementioned activated adsorption sites. With increasing pH, more activated adsorption sites are deprotonated, which in turn results in gradual increase of metal ions removal efficiency (Li et al., 2005; Miretzky and Cirelli, 2009; Pillai et al., 2009). 3.2.2. Isothermal adsorption equilibrium study For a better understanding of adsorption mechanism and the interactions between adsorbate and adsorbent, adsorption isothermal equilibrium experiments for AG25 and Hg2þ were conducted at 298 K and pH of 7.0 and 5.0 respectively. The results are shown in Fig. 4a and b. The adsorption behaviors for both AG25 and Hg2þ display similar change tendencies which increase sharply as the initial concentrations increasing at the beginning. The equilibrium dye and metal ions uptakes of CS-Gln-MCM are much higher than those of CS-MCM, indicating that functional groups on CS-Gln-MCM introduced from glutamine bring an evident enhancement in the adsorption capacities. In order to further study the adsorption behaviors of the magnetic microspheres, two kinds of widely used isothermal models, Langmuir (1918) and Freundlich model (Freundlich, 1906), were employed to analyze the obtained adsorption equilibrium data respectively. The detailed equations of aforementioned two models are described in Supporting Information Text S1. The simulated results based on the aforementioned models are listed in Table 1. According to Table 1, the correlation coefficients (R2) of the nonlinear form for Langmuir model are higher than those of Freundlich model for both AG25 and Hg2þ by the two magnetic adsorbents. Besides, they are usually the monolayer

26

X. Tao et al. / Environmental Pollution 209 (2016) 21e29

160

700

(a)

600

120

500

100

qe (mg.g-1)

-1

qe (mg.g )

(b)

140

400 300 200 CS-MCM

100

80 60 CS-MCM CS-Gln-MCM

40 20

CS-Gln-MCM

0

0 0

100

200

300

400

500

0

600

50

100

Ce (mg.dm ) 700 600

140

(c)

250

300

350

(d)

100

400

-1

qt (mg.g )

-1

200

120

500

qt (mg.g )

150

Ce (mg.dm-3)

-3

300 CS-MCM CS-Gln-MCM

200

80 60 40

100

20

0

0 0

20

40

60

80

CS-MCM CS-Gln-MCM 0

100

20

40

60

80 100 120 140 160 180 200

t (min)

t (min)

Fig. 4. Adsorption isotherms (a, b) and kinetics (c, d) of magnetic adsorbents for removal of AG25 (B/C in a and c) and Hg2þ (,/- in b and d) onto CS-MCM/CS-Gln-MCM in respective single pollutant system.

adsorptions for not only Hg2þ by chelation effect but also AG25 through hydrogen bonding or electrostatic interactions binding to the chitosan-based adsorbents. As a result, the adsorption behaviors of CS-Gln-MCM and CS-MCM for aforementioned two pollutants could be both described by Langmuir isothermal model better, indicating a monolayer adsorption of metal ions and dye on the relatively homogeneous surface of the magnetic microspheres with approximately identical energy. 3.2.3. Adsorption kinetics analysis Adsorption kinetics is another one of the most important aspects to well understand the whole adsorption process and mechanism besides adsorption isotherms. The adsorption kinetics experiments of CS-MCM and CS-Gln-MCM for AG25 and Hg2þ were carried out at 298 K and pH 7.0 for AG25 and 5.0 for Hg2þ. The results are displayed in Fig. 4c and d.

The adsorption kinetics curves of AG25 and Hg2þ are quite similar as contact time prolonged. It is observed that the two magnetic microspheres soon achieve adsorption equilibrium in no more than 20.0 min, indicating a high removal efficiency of the chitosan-based magnetic adsorbents, which is of great significance in practical applications. Theoretic fitting analysis for the kinetics data based on pseudofirst order (Lagergren, 1898), pseudo-second order (Ho and McKay, 1998), and intra-particle diffusion model (Cheung et al., 2000) was also carried out to explore their adsorption mechanism further. The detailed equations of aforementioned three models are described in Supporting Information Text S2. All the experimental data were fitted by aforementioned kinetics models, and their parameters are all listed in Table 2. According to Table 2, R2 values of pseudo-second order model for both AG25 and Hg2þ are higher than those of other two models.

Table 1 The comparison of the Langmuir and Freundlich adsorption constants obtained from adsorption isotherms of AG25 and Hg2þ onto CS-MCM and CS-Gln-MCM in respective single system at 298 K. Adsorbents

CS-MCM CS-Gln-MCM

Pollutants

AG25 Hg2þ AG25 Hg2þ

pH

7.0 5.0 7.0 5.0

qm,exp (mg g1)

355.14 73.32 652.41 141.10

Langmuir model

Freundlich model

qm (mg g1)

b  102 (dm3 mg1)

R2

Kf

n

R2

408.79 103.11 698.95 199.23

0.28 1.08 0.20 0.69

0.982 0.965 0.994 0.986

10.57 6.969 12.96 7.030

1.93 2.354 1.74 1.940

0.921 0.912 0.972 0.947

X. Tao et al. / Environmental Pollution 209 (2016) 21e29

Meanwhile the theoretical qe values obtained by non-liner fitting with pseudo-second order model are more close to the experimental values (qm,exp), meaning that the adsorption kinetics of both CS-Gln-MCM and CS-MCM could be better described by pseudosecond order equation. The chemisorption could be considered as the rate controlling mechanism in adsorption of AG25 and Hg2þ onto both magnetic adsorbents, which are fully consistent with those drawn from adsorption isotherm analysis as mentioned above. 2þ

3.3. Simultaneous adsorption of AG25 and Hg component system

800

CS-MCM CS-Gln-MCM

qe (mg.g )

400 2+

Hg

CS-MCM CS-Gln-MCM

100 50 0 2.0

The simultaneous adsorption experiments were carried out in mixed aqueous solutions containing both AG25 and Hg2þ under different pH levels ranged from 2.0 to 5.5. The initial concentrations of AG25 and Hg2þ are both kept at 1.5 mmol dm3. The final results are presented in Fig. 5. It is found that the pH dependences of both AG25 and Hg2þ uptakes are quite similar to those in their respective single system shown in Fig. 3 but AG25 and Hg2þ uptakes both decrease. However, the adsorption capacity for Hg2þ shows a higher decrease than that for AG25 at lower pH, indicating an evident impact to Hg2þ adsorption under the strong acidic conditions. To study the adsorption behavior in the binary pollutant system quantitatively, the adsorption selectivity of CS-Gln-MCM toward a certain adsorbate is characterized by a selectivity coefficient of a (Li et al., 2005, 2015), which is calculated based on the following equation (Eq. (3)) and listed in Table 3.

aAB ¼

AG25

600

in binary

27

qe;A Ce;B qe;B Ce;A

2.5

3.0

3.5

4.0

4.5

5.0

5.5

pH Fig. 5. Effects of initial solution pH on AG25 (B/C) and Hg2þ (,/-) uptakes onto CSMCM/CS-Gln-MCM in the binary pollutant system.

Table 3 2þ Selectivity coefficients ðaAG25 in Hg Þ of CS-MCM and CS-Gln-MCM for AG25 over Hg binary pollutant system at 298 K and various pH levels. Adsorbent

CS-MCM CS-Gln-MCM

aAG25 Hg

pH 2.37

3.11

4.12

5.09

29.58 234.06

7.10 7.95

1.54 3.55

0.61 1.28

et al., 2015; Ngah et al., 2011). However, the positive surface of absorbents limits the adsorption of Hg2þ. Accordingly, the increase of acidic level would promote AG25 uptakes but inhibit the Hg2þ adsorption. In addition, strong acidic eluents may be used for separation and selective recovery of Hg2þ and AG25 after adsorption treatment by this composite adsorbent. On the other hand, as solution pH increases, the surface charges become less positive due to the deprotonation. AG25 was removed mainly through the interactions between the secondary amine groups of dye and carboxyl groups on glutamine especially at higher pHs. At the same time, Hg2þ uptakes linearly increase from pH 2.0 to 5.0 and then gradually reach to the platform because of the chelating effects between the Hg2þ and the amino or hydroxyl groups of chitosan as well as the interactions to the additional carboxyl and amide groups on glutamine backbone (Li et al., 2015; Miretzky and Cirelli, 2009; Rinaudo, 2006). Therefore AG25 and Hg2þ could be both effective removal at weak acidic conditions for their different affinities to various adsorption active sites on CS-Gln-MCM.

(3)

where aAB is the selectivity factor of adsorbate A over adsorbate B, qe,A and qe,B is the equilibrium adsorption capacity of A and B, respectively. Ce,A and Ce,B is the equilibrium concentration of A and B, respectively. Just as the a values suggested from Table 3, both chitosanbased magnetic microspheres show specific AG25 adsorption in strongly acidic solutions, but the selectivity is greatly weakened as pH increased. Moreover, the selective adsorption of CS-GlnMCM for dye is obviously enhanced in comparison to that of CS-MCM, which may be due to the introduction of additional functional groups from grafted glutamine. However, there is almost no specific selectivity at pH 5.0 (weak acidic level) and CS-Gln-MCM could be simultaneous and efficient removal of both AG25 and Hg2þ from their mixture. These results suit the expectations based on the pH dependence of the adsorption behaviors of CS-Gln-MCM and CS-MCM for those two pollutants. At strong acidic conditions, the surface of the adsorbents turn more positive for the formation of much more eNHþ 3 groups on the chitosan backbone, which increases their affinity to the anionic sulfate groups of AG25. Furthermore, the secondary amine groups of dye and carboxyl groups on glutamine could easily form hydrogen bonding (Crini and Badot, 2008; Kyzas

3.4. Desorption and reusability study Recycling and reusability of an adsorbent is very important for further practical applications as well as the reduction of cost. In this

Table 2 Kinetic parameters of AG25 and Hg2þ onto CS-MCM and CS-Gln-MCM in respective single system at 298 K. Adsorbents

Pollutants pH qm,exp (mg g1) Pseudo-first order model

Pseudo-second order model

qe1,cal (mg g1) k1 (min1) R2 CS-MCM

AG25 Hg2þ CS-Gln-MCM AG25 Hg2þ

7.0 355.14 5.0 73.32 7.0 652.41 5.0 141.10

208.44 67.23 606.46 123.34

0.3121 0.6543 0.2856 0.8743

qe2

cal

Intraparticle diffusion model

(mg g1) k2  104 (g mg1 min1) R2

0.967 412.54 0.947 70.90 0.983 660.54 0.964 128.51

11.5 16.30 6.41 13.51

C (mg g1) kp (mg g1 min0.5) R2

0.984 177.73 0.985 46.23 0.985 265.33 0.986 89.38

32.22 3.01 54.17 5.02

0.500 0.299 0.566 0.257

28

X. Tao et al. / Environmental Pollution 209 (2016) 21e29

Table 4 Recovery efficiency of CS-Gln-MCM after adsorption of AG25 and Hg2þ in respective single pollutant system. Adsorbates

Cycle Cycle Cycle Cycle Cycle

I II III IV V

Hg2þ

AG25 Adsorption capacity (mg g1)

Recovery efficiency (%)

Adsorption capacity (mg g1)

Recovery efficiency (%)

663.55 601.81 585.66 567.17 543.92

98.91 89.71 87.31 84.55 81.08

140.45 137.98 131.26 134.42 132.40

99.54 97.79 93.03 95.27 93.84

current work, a dilute EDTA aqueous solution was employed for the recovery of Hg-loaded adsorbents, since EDTA is an effective chelating agent and metal ions could be easily substituted and separated from original adsorbents (Mishra, 2014). At the same time, 0.1 mol dm3 NaOH is chosen to recover AG25-loaded adsorbents. The recovery efficiencies of CS-Gln-MCM for AG25 and Hg2þ are listed in Table 4. From Table 4, the recovery efficiencies for various pollutants are both higher than 80% after five cycles, indicating that CS-Gln-MCM could be easily regenerated and reused which is fully qualified in practical applications. 4. Conclusion The amino acid of glutamine, a green material, was chosen to modify chitosan, thus a kind of biodegradable adsorbents has been prepared by only one step. Glutaraldehyde is used as crosslinker and connecting reagent here. The grafting glutamine and crosslinking with chitosan occur simultaneously through amine aldehyde reaction. This modification greatly improves the adsorption efficiency of the chitosan-based magnetic adsorbent for both AG25 and Hg2þ from water. Moreover, aforementioned two pollutants could be efficiently removed together in their aqueous mixtures at weak acidic conditions. Besides the contributions from chitosan, the enhanced adsorption efficiency for AG25 is still due to hydrogen bonding effect between secondary amine of AG25 and carboxyl groups of glutamine. While the improved Hg2þ uptake is attributed to the formation of effective interactions between Hg2þ and the carboxyl and amide groups of glutamine for chelating effects. According to the further investigation of the adsorption mechanism including adsorption isotherms and kinetics, the adsorption of AG25 and Hg2þ by CS-MCM and CS-Gln-MCM both follows a homogeneous monolayer chemisorption process. In addition, the rapid magnetic separation as well as efficient recovery and reuse of used adsorbents after saturated adsorption from aqueous solutions greatly guarantee the possibility of the practical applications in wastewater treatment for both chitosan-based adsorbents. Acknowledgments Supported by the Natural Science Foundation of China (Grant Nos. 51438008 and 51378250). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2015.11.020. References Al-Ghouti, M.A., Li, J., Salamh, Y., Al-Laqtah, N., Walker, G., Ahmad, M.N., 2010. Adsorption mechanisms of removing heavy metals and dyes from aqueous solution using date pits solid adsorbent. J. Hazard. Mater. 176, 510e520.

Ambashta, R.D., Sillanpaa, M., 2010. Water purification using magnetic assistance: a review. J. Hazard. Mater. 180, 38e49. Anliker, R., 1979. Ecotoxicology of dyestuffsda joint effort by industry. Ecotox. Environ. Safe 3, 59e74. Banat, I.M., Nigam, P., Singh, D., Marchant, R., 1996. Microbial decolorization of textile-dye-containing effluents: a review. Bioresour. Technol. 58, 217e227. Chen, R., 2010. Development of auxiliaries in low-carbon economy era. J. Dye Finish 36, 46e51. Cheung, C.W., Porter, J.F., McKay, G., 2000. Sorption kinetics for the removal of copper and zinc from effluents using bone char. Sep. Purif. Technol. 19, 55e64. Chiou, M.S., Chuang, G.S., 2006. Competitive adsorption of dye metanil yellow and RB15 in acid solutions on chemically cross-linked chitosan beads. Chemosphere 62, 731e740. Chmielewska, E., 2008. Development of new generation of environmental adsorbents based on natural nanomaterials. Chem. Listy 102, 124e130. Correia, V.M., Stephenson, T., Judd, S.J., 1994. Characterization of textile wastewaters e a review. Environ. Technol. 15, 917e929. Crini, G., 2005. Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment. Prog. Polym. Sci. 30, 38e70. Crini, G., Badot, P.M., 2008. Application of chitosan, a natural aminopolysaccharide, for dye removal from aqueous solutions by adsorption processes using batch studies: a review of recent literature. Prog. Polym. Sci. 33, 399e447. Dabrowski, A., 2001. Adsorption e from theory to practice. Adv. Colloid Interface Sci. 93, 135e224. Dai, J., Yan, H., Yang, H., Cheng, R.S., 2010. Simple method for preparation of chitosan/poly(acrylic acid) blending hydrogel beads and adsorption of copper(II) from aqueous solutions. Chem. Eng. J. 165, 240e249. Dasgupta, J., Sikder, J., Chakraborty, S., Curcio, S., Drioli, E., 2015. Remediation of textile effluents by membrane based treatment techniques: a state of the art review. J. Environ. Manag. 147, 55e72. Deng, J.H., Zhang, X.R., Zeng, G.M., Gong, J.L., Niu, Q.Y., Liang, J., 2013. Simultaneous removal of Cd(II) and ionic dyes from aqueous solution using magnetic graphene oxide nanocomposite as an adsorbent. Chem. Eng. J. 226, 189e200. Doke, S.M., Yadav, G.D., 2014. Novelties of combustion synthesized titania ultrafiltration membrane in efficient removal of methylene blue dye from aqueous effluent. Chemosphere 117, 760e765. Elsasser, V.H., 2005. Textiles Concepts and Principles. Fairchild Publications, New York, pp. 35e40. Freundlich, H., 1906. Concerning adsorption in solutions. Z. Phys. Chem. 57, 385e470. Fu, F., Han, W., Tang, B., Hu, M., Cheng, Z., 2013. Insights into environmental remediation of heavy metal and organic pollutants: simultaneous removal of hexavalent chromium and dye from wastewater by zero-valent iron with ligand-enhanced reactivity. Chem. Eng. J. 232, 534e540. Gupta, V.K., Suhas, 2009. Application of low-cost adsorbents for dye removal e a review. J. Environ. Manag. 90, 2313e2342. Ho, Y.S., McKay, G., 1998. Sorption of dye from aqueous solution by peat. Chem. Eng. J. 70, 115e124. Hoffman, R.V., 2004. Organic Chemistry: an Intermediate Text. John Wiley & Sons Inc, Hoboken. Jarup, L., 2003. Hazards of heavy metal contamination. Br. Med. Bull. 68, 167e182. Kyzas, G.Z., Siafaka, P.I., Pavlidou, E.G., Chrissafis, K.J., Bikiaris, D.N., 2015. Synthesis and adsorption application of succinyl-grafted chitosan for the simultaneous removal of zinc and cationic dye from binary hazardous mixtures. Chem. Eng. J. 259, 438e448. Lagergren, S., 1898. About the theory of so-called adsorption of soluble substances. Kung. Sven. Veten. Hand. 24, 1e39. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361e1403. Li, H., Fu, L., 2014. Research progress of wastewater advanced treatment technology for printing and dyeing industrial parks. Environ. Eng. 32, 18e21. Li, K., Wang, Y.W., Huang, M., Yan, H., Yang, H., Xiao, S.J., Li, A.M., 2015. Preparation of chitosan-graft-polyacrylamide magnetic composite microspheres for enhanced selective removal of mercury ions from water. J. Colloid Interface Sci. 455, 261e270. Li, N., Bai, R.B., Liu, C.K., 2005. Enhanced and selective adsorption of mercury ions on chitosan beads grafted with polyacrylamide via surface-initiated atom transfer radical polymerization. Langmuir 21, 11780e11787. Martinez-Huitle, C.A., Brillas, E., 2009. Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review. Appl.

X. Tao et al. / Environmental Pollution 209 (2016) 21e29 Catal. B Environ. 87, 105e145. Miretzky, P., Cirelli, A.F., 2009. Hg2þ removal from water by chitosan and chitosan derivatives: a review. J. Hazard. Mater. 167, 10e23. Mishra, S.P., 2014. Adsorption-desorption of heavy metal ions. Curr. Sci. 107, 601e612. Monteiro Paschoal, F.M., Anderson, M.A., Zanoni, M.V.B., 2009. Simultaneous removal of chromium and leather dye from simulated tannery effluent by photoelectrochemistry. J. Hazard. Mater. 166, 531e537. Ngah, W.S.W., Teong, L.C., Hanafiah, M.A.K.M., 2011. Adsorption of dyes and heavy metal ions by chitosan composites: a review. Carbohydr. Polym. 83, 1446e1456. Nidheesh, P.V., Gandhimathi, R., Ramesh, S.T., 2013. Degradation of dyes from aqueous solution by Fenton processes: a review. Environ. Sci. Pollut. R. 20, 2099e2132. Pillai, C.K.S., Paul, W., Sharma, C.P., 2009. Chitin and chitosan polymers: chemistry, solubility and fiber formation. Prog. Polym. Sci. 34, 641e678. Reddy, D.H.K., Lee, S.M., 2013. Application of magnetic chitosan composites for the removal of toxic metal and dyes from aqueous solutions. Adv. Colloid Interface Sci. 201, 68e93. Rinaudo, M., 2006. Chitin and chitosan: properties and applications. Prog. Polym. Sci. 31, 603e632. Robinson, T., McMullan, G., Marchant, R., Nigam, P., 2001. Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative. Bioresour. Technol. 77, 247e255. Saroj, S., Kumar, K., Pareek, N., Prasad, R., Singh, R.P., 2014. Biodegradation of azo

29

dyes Acid Red 183, Direct Blue 15 and Direct Red 75 by the isolate Penicillium oxalicum SAR-3O. Chemosphere 107, 240e248. Tastan, B.E., Ertugrul, S., Donmez, G., 2010. Effective bioremoval of reactive dye and heavy metals by Aspergillus versicolor. Bioresour. Technol. 101, 870e876. Thakur, V.K., Thakur, M.K., 2014. Recent advances in graft copolymerization and applications of chitosan: a review. ACS Sustain. Chem. Eng. 2, 2637e2652. Vimonses, V., Jin, B., Chow, C.W.K., 2010. Insight into removal kinetic and mechanisms of anionic dye by calcined clay materials and lime. J. Hazard. Mater. 177, 420e427. Wang, J., Zheng, S., Shao, Y., Liu, J., Xu, Z.Y., Zhu, D.Q., 2010. Amino-functionalized Fe3O4@SiO2 coreeshell magnetic nanomaterial as a novel adsorbent for aqueous heavy metals removal. J. Colloid Interface Sci. 349, 293e299. Wang, W., Tian, G., Zhang, Z., Wang, A., 2015. A simple hydrothermal approach to modify palygorskite for high-efficient adsorption of methylene blue and Cu(II) ions. Chem. Eng. J. 265, 228e238. WHO, 2011. Guidelines for Drinking-Water Quality. World Health Organization, p. 564. Xin, J.H., 2006. A review of the textile coloration industry in China. Color Technol. 33, 59e71. Yan, H., Yang, L.Y., Yang, Z., Yang, H., Li, A.M., Cheng, R.S., 2012. Preparation of chitosan/poly(acrylic acid) magnetic composite microspheres and applications in the removal of copper(II) ions from aqueous solutions. J. Hazard. Mater. 229e230, 371e380.