Journal of Colloid and Interface Science 455 (2015) 125–133

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Synthesis of a novel ionic liquid modified copolymer hydrogel and its rapid removal of Cr (VI) from aqueous solution Yinhua Jiang ⇑, Fan Li, Guibing Ding, Yecheng Chen, Yan Liu, Yuanzhi Hong, Peipei Liu, Xiuxiu Qi, Liang Ni School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, PR China

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

a r t i c l e

i n f o

Article history: Received 7 March 2015 Accepted 18 May 2015 Available online 22 May 2015 Keywords: Copolymer hydrogel Ionic liquid modification Rapid adsorption Cr (VI)

⇑ Corresponding author. Fax: +86 0511 88791708. E-mail address: [email protected] (Y. Jiang). http://dx.doi.org/10.1016/j.jcis.2015.05.030 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

a b s t r a c t A novel ionic liquid modified copolymer hydrogel (PAMDA) was successfully synthesized by a simple water solution copolymerization using acrylamide (AM), dimethyldiallylammonium chloride (DADMAC) and ionic liquid (1-allyl-3-methylimidazolium chloride; [Amim]Cl) as copolymerization monomers. The structure and morphology of as-prepared copolymer hydrogel PAMDA were confirmed by Fourier transform infrared (FT-IR), field-emission scanning electron microscope (FE-SEM) and thermogravimetric analysis (TG). The copolymer hydrogel was applied as a novel adsorbent for the rapid removal of Cr (VI) from aqueous solution. The effects of several parameters such as the content of ionic liquid [Amim]Cl, solution pH, contact time, adsorbent dosage and initial Cr (VI) concentration on the adsorption were also investigated. The modification of [Amim]Cl significantly enhanced Cr (VI) adsorption. The adsorption equilibrium data fitted with Langmuir isotherm model better than Freundlich isotherm model. The maximum adsorption capacity for Cr (VI) ions was 74.5 mg L1 at 323 K based on Langmuir isotherm model. The removal rate could reach 95.9% within 10 min at 323 K and the adsorption process of Cr (VI) on PAMDA was well described by the pseudo-second-order kinetic model. The activation energy of adsorption was further investigated and found to be 1.094 kJ mol1, indicating the adsorption of Cr (VI) onto PAMDA was physisorption. Ó 2015 Elsevier Inc. All rights reserved.

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1. Introduction Due to the rapid industrial development, water contamination by heavy metals has become more and more serious due to the excessive release of industrial effluents [1–12]. Heavy metals such as cadmium, chromium, copper, lead and mercury are highly dangerous to human beings as well as environment, even at low concentration levels in water [13]. As one of the most hazardous heavy metals, chromium is often detected in industrial wastewaters originating from many industries such as electroplating, dying, metal polishing, chromic salts industry, textile and leather tanning [1]. In aquatic environment, chromium primarily exists in two stable oxidation states, Cr (III) and Cr (VI) [4]. Cr (VI) has a high mobility in aquatic system and is about 500 times more toxic than Cr (III) [10]. It has been reported that Cr (VI) is highly carcinogenic, mutagenic and teratogenic to living organisms including human beings [10,14]. The US Environmental Protection Agency (EPA) has defined Cr (VI) as one of the top priority toxic pollutants, and the permissible limit of Cr (VI) in drinking water is 0.05 mg L1 [11]. Therefore, the removal of highly toxic Cr (VI) from natural water sources and industrial wastewaters is one of the biggest challenges in remediation of contaminated aquatic environments. In the past decades, various treatment technologies have been proposed for the removal of Cr (VI), including chemical precipitation, adsorption, ion exchange, ultrafiltration and solvent extraction. Among these techniques, adsorption has been recognized as a more promising process for removal of Cr (VI) from aquatic environment due to its low cost, availability, ease of operation and high efficiency [13,15]. A wide variety of adsorbents such as activated carbons [10], polymers [14], biomass [16,17], gels [18], natural and waste materials [17,19], and clay minerals [20], have been synthesized for the treatment of Cr (VI)-contaminated wastewater. However, the development of novel adsorbents with low-cost, high adsorption capacity, easy separation and fast adsorption kinetic for Cr (VI) species from wastewaters is still a continuing goal of environmental remediation efforts. Polyacrylamide (PAM), a water-soluble polymer with large numbers of amide side groups, has been widely employed as adsorbents and flocculants for the purification of industrial effluents and tap water, due to its non-toxicity, high efficiency and relatively low cost [21–23]. The amide side groups in PAM chains are the efficiently active sites to enrich heavy metal ions from aquatic solutions by forming stable metal ion–amide linkages [22]. In addition, various functional groups like amino, ammonium, carboxyl, carboxymethyl or hydroxyl group can also be introduced into the PAM chains by copolymerization or grafting method. These functional groups presenting in the PAM structure can provide binding sites to remove heavy metals from aqueous solutions more efficiently [24–26]. For instance, Karimi and coworkers prepared ethylenediamine modified spherical silica–polyacrylamide for the removal of hexavalent chromium and observed that the modification with ethylenediamine greatly enhanced the adsorption capacity of composite [25]. Wisniewska et al. investigated the impact of polyacrylamide with different contents of carboxyl groups on the chromium (III) oxide adsorption and proved that PAM with the carboxyl group content of 50% was the most effective flocculant for chromium (III) oxide suspension [26]. However, in aqueous solution the physical and chemical properties of heavy metals are very different, so it is very critical to introducing appropriate type and content of functional groups to PAM chains to improve the adsorption capacity and selectivity of PAM. For Cr (VI), it exists in water and wastewater mainly in  2 the form of Cr2O2 and HCr2O 7 , HCrO4 , CrO4 7 [27]. Therefore, PAM modified with positively charged functional groups (cationic PAM) is expected to be a more effective adsorbent for the adsorption of Cr (VI) anions from aqueous solution.

Dimethyldiallylammonium chloride (DADMAC), a hydrophilic compound with a quaternary ammonium group, can be polymerized to its corresponding polymer, which is widely used in water treatment, paper manufacturing, mining, and life science applications due to its highly hydrophilic charged quaternary ammonium groups [28]. Moreover, DADMAC is also a suitable monomer to synthesize cationic PAM copolymer by the copolymerization of DADMAC and acrylamide (AM). Zheng and co-workers have reported the preparation of Poly (dimethyldiallylammonium chloride)/polyacrylamide hydrogel and used it as an adsorbent to remove nitrate anion from aqueous solution [29]. Our previous work has also demonstrated the synthesis of poly (AM co DADMAC)/silica sol and its efficient adsorption of methyl orange, which is partly due to the introduction of silica sol to the poly (AM co DADMAC) structure [30]. However, with the introduction of inorganic silica sol, it is difficult to obtain homogeneous hydrogel. Recently, ionic liquids (ILs) defined as an organic/inorganic salt have gained significant interest due to high ionic conductivity, high chemical and electrochemical stability, low melting point, non-flammability and negligible vapor pressure [9,31]. Introducing ionic liquid monomer into polymer backbones could not only enable copolymer inherit the features of ionic liquids, but also create some special properties [32,33]. The synergetic properties of Poly (ionic liquid) and hydrogel might render copolymer new functionalities and hence broaden the possibilities of its application in environmental cleaning. To the best of our knowledge, there are no investigations reporting the synthesis of poly (AM co DADMAC) containing ionic liquid and its adsorption of Cr (VI) anions from aqueous solution. In this study, a novel Poly (AM-co-[Amim]Cl-co-DADMAC) hydrogel (PAMDA) was prepared by a simple water solution copolymerization using an imidazole-containing ionic liquid 1-allyl-3-methylimidazolium chloride ([Amim]Cl), DADMAC and AM as comonomers. The adsorption property of copolymer PADMA was studied by adsorbing Cr (VI) anions from aqueous solution. The effects of ionic liquid [Amim]Cl content, solution pH, contact time, initial Cr (VI) concentration and temperature on Cr (VI) adsorption were systematically investigated. From a technological point of view, the greatest advantages of using copolymer hydrogels (PAMDA) as adsorbent material for heavy metals are the simplicity of synthesis, fast adsorption, low-cost and non-toxicity of the final material. 2. Materials and methods 2.1. Materials N,N0 -methylenebisacrylamide (MBA), acrylamide (AM), 1,5-diphenylcarbazide (AR) and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. 2,20 -Azo-bisisobutyronitrile (AIBN) was bought from Tianjin Guangfu Fine Chemical Research Institute. 1-allyl-3-methylimidazolium chloride ([Amim]Cl) was obtained from Hao Rui Chemicals (Shanghai) Co., Ltd. Dimethyldiallylammonium chloride (DADMAC) was purchased from the new Haitian biological Technology Ltd. Potassium chromate (K2CrO4) used for preparing the stock solution of Cr (VI) ions was supplied by Jinlian Fine Chemical plant in Shanghai. All solutions used were prepared with deionized water. 2.2. Preparation of poly (AM co DADMAC) (PAMD) and PAMDA hydrogel A series of copolymer hydrogel (PAMDA) samples with different weight percentages of ionic liquid [Amim]Cl were prepared using a simple water solution copolymerization method by the following procedure. In brief, 4 g AM and 2 g DADMAC were dissolved in 10 mL deionized water, together with various weights

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Scheme 1. The schematic route for the synthesis of PAMDA followed by its Cr (VI) anion adsorption.

of [Amim]Cl. 0.014 g MBA was then added into the solution as cross-linker of the free-radical polymerization, followed by the dropwise addition of 0.06 g AIBN initiator in absolute ethanol (10 mL). Next, the mixture was stirred for a few minutes and polymerized at 70 °C for 1.5 h. Finally, the formed hydrogels were cut into small pieces with the size of approximately 1 mm and dried in vacuum oven at 80 °C for 24 h, and then pulverized for further use. For comparison, poly (AM co DADMAC) hydrogel (PAMD) was prepared via the same synthetic procedure to that of PAMDA but without [Amim]Cl. The schematic route for the synthesis of PAMDA followed by Cr (VI) anions adsorption is shown in Scheme 1. 2.3. Characterizations Fourier transform infrared (FTIR) spectra of hydrogels PAMDA were recorded on a FTIR Spectrophotometer (Thermo Nicolet, NEXUS, TM) in the range of 4000–500 cm1 using KBr pellet. The thermogravimetric analysis (TG) was carried out using a Netzsch thermal analysis TG/DTA system. Measurements were made heating from 20 to 1000 °C, at a heating rate of 10 °C min1 under N2 atmosphere. The morphology of the hydrogel was characterized with field-emission scanning electron microscopy (FE-SEM, Philips FEI XL30). The analysis of Cr (VI) ion concentration in aqueous samples was determined using a spectrophotometer (TU-1810 UV–Vis, Purkinje General Instrument Co., Ltd. in Beijing) at 540 nm after complexation with 1,5-diphenylcarbazide [34]. 2.4. Adsorption procedure

C0 V 1  CeV 2 m

3. Results and discussion 3.1. FTIR spectra of adsorbents The FTIR spectra of PAMD and PAMDA with 5 wt.% ionic liquid [Amim]Cl are shown in Fig. 1. In the FTIR spectrum of PAMD, two peaks at 3021 cm1 and 998 cm1 are attributed to the presence of the quaternary amine group [29,36]. The characteristic absorption bands at 2932 cm1 and 2856 cm1 belong to methylene stretching and bending vibrations. The peaks at 1680 cm1 PAMD

In order to evaluate the adsorption property of the as-prepared hydrogel PAMDA modified with different amounts of [Amim]Cl, batch adsorption experiments were carried out by adding 0.1 g as-prepared PAMDA into 50 mL of 100 mg L1 Cr (VI) solution with constant stirring at 303 K until the adsorption equilibrium was reached. After equilibration, the adsorbents were removed from suspension by centrifuging at 6000 rpm for 20 min and the supernatant was analyzed for surplus Cr (VI) concentration by UV–Vis spectrophotometer. The adsorption capacity of PAMDA for Cr (VI) was calculated by the following equation [35]:

q¼

where q (mg g1) represents the amount of Cr (VI) adsorbed onto PAMDA; C0 (mg L1) is the initial Cr (VI) concentration, Ce (mg L1) is the equilibrium Cr (VI) concentration, m (g) is the mass of PAMDA, V1 and V2 (L) are the volumes of Cr (VI) solutions before and after the adsorption. The effects of solution pH, contact time, adsorbent dosage and initial Cr (VI) concentration on the adsorption capacity of PAMDA with 5 wt.% ionic liquid [Amim]Cl were systemically investigated. The effect of solution pH on the removal of Cr (VI) anion was studied in the pH range of 2–7.3 at 303 K with the aid of HCl or NaOH solution (0.1 and 1.0 mol L1). The dosage of adsorbent was varied from 0.4 to 2.4 g L1. For isotherm studies, the experiments were carried out at three different temperatures with initial concentration of Cr (VI) ranging from 20 to 120 mg L1. Kinetic data were obtained by conducting experiments at three different temperatures by varying contact time from 1 to 50 min.

3021

1606

PAMDA 2856 3320

998 1174 765

2932 3180 1680

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumbers ( cm )

ð1Þ Fig. 1. The FTIR spectra of PAMD and PAMDA with 5 wt.% [Amim]Cl.

Y. Jiang et al. / Journal of Colloid and Interface Science 455 (2015) 125–133

and 1606 cm1 correspond to C@O stretching vibration and NAH deformation vibrations, respectively. As compared to PAMD, two new peaks at 765 cm1 and 1174 cm1, which are respectively assigned to the out-of-plane and in-plane bending vibrations of CAH in the imidazolium groups, are observed in the FTIR spectrum of PAMDA [37]. The presence of two new bands confirms that [Amim]Cl has successfully copolymerized to PAMDA chains. Moreover, the intensities of two peaks at 3021 cm1 and 998 cm1 shows obviously increase, indicating that the positive charge density of PAMDA chains is increased by copolymerizing with ionic liquid [Amim]Cl.

90

Cr(VI) Removal (%)

128

85 80 75 70 65 60 0

3.2. FE-SEM and TG analysis FE-SEM is widely used to study the morphological and surface characteristics of the adsorbent materials. The surface morphologies of PAMD and copolymer PAMDA containing 5 wt.% ionic liquid [Amim]Cl are shown in Fig. 2. It can be observed that PAMD (Fig. 2a) presents a compact structure with low porosity and a smooth surface. After copolymerizing with 5 wt.% [Amim]Cl, copolymer PAMDA (Fig. 2b) exhibits a relatively loose structure with a rough surface and high porosity instead. This structural change may result from the strong repulsions among charged groups and the less orderly packing of the polymer chains due to the bulky imidazole groups within the macromolecules. And in terms of ion adsorption, this kind of structure should be favor to the adsorption of ions. Thermal stability of PAMDA is very important for its practically application as an adsorbent, and TG analysis (see Fig. S1) shows that [Amim]Cl modification has little affect on its thermal stability. The total weight loss can be roughly divided into four regions. The first weight loss appearing below 200 °C is due to the loss of adsorbed water. The second weight loss in the temperature range of 200–320 °C corresponds to NH3 evolution. The sharpest decreases in weight loss extrapolated to be 480 °C and a small weight loss after 480 °C are due to the main chain scission of polymers. However, from the curves, it can be found that the first weight loss of PAMDA (14.9%) is a little higher than that of PAMD (10.6%), which may result from the improvement in hydrophilicity of copolymer PAMDA after [Amim]Cl copolymerization. 3.3. Effect of the [Amim]Cl content on Cr (VI) removal The effect of different weight percentages of [Amim]Cl on Cr (VI) removal is shown in Fig. 3. From Fig. 3, it is obvious that the removal rate of Cr (VI) on PAMDA after [Amim]Cl modification is higher than that on PAMD (60.91%). This result proves that the rough, porous and loose structure of copolymer PAMDA can enhance the Cr (VI) adsorption. Moreover, [Amim]Cl modification improves the surface positive charge density, which leads to an increase in Cr (VI) adsorption via the strong electrostatic

1

2

3

4

5

6

7

[Amim]Cl Content (wt%) Fig. 3. Effect of [Amim]Cl contents on the Cr (VI) removal of PAMDA (initial Cr (VI) concentration: 100 mg L1; T: 303 K; adsorbent dosage: 0.1 g; t = 20 min).

interaction. Fig. 3 also shows that the amount of [Amim]Cl has a direct effect on the removal of Cr (VI). The removal percentage of Cr (VI) increases with the increase of [Amim]Cl weight from 0.5 to 5 wt.% and reaches to the maximum of 90.6% at 5 wt.% [Amim]Cl. The increased amount of positively charged [Amim]Cl in the copolymer structure provides more adsorption binding sites for Cr (VI) anions, so the adsorption amount of Cr (VI) anions is enhanced. When the [Amim]Cl content is further increased to 7 wt.%, a slight decline in the removal capacity for Cr (VI) is observed. A similar result was described by Zheng and collaborator in the case of nitrate adsorption onto poly (dimethyldiallylammonium chloride)/polyacrylamide hydrogel [29]. Hence, in the subsequent adsorption experiments, PAMDA with 5 wt.% [Amim]Cl was chosen for the analysis and optimization of operating conditions for Cr (VI) adsorption. 3.4. Effect of solution pH The solution pH is an important parameter which strongly affects the adsorption of heavy metals onto the solid–liquid interface [38]. The relations between the initial pH of solution and the removal percentage of Cr (VI) anions, and the Cr (VI) anion adsorption amount onto PAMDA, respectively, are shown in Fig. 4. In the pH range of 2–7.3, the removal percentage of Cr (VI) anions decreases with the increase of pH and the maximum adsorption occurred at pH = 2 (90.6%). The corresponding adsorption capacity of PAMDA is 45.3 mg g1 at pH = 2, which drops to 20.9 mg g1 at pH = 7.3. The above result could be proven by taking the point of zero charge (pHPZC) of the hydrogel surface and the forms of Cr (VI) ions in aqueous solution of different pH value into account [39]. The pHPZC value of copolymer is the point of intersection of the curve (pH0–pHf) versus pH0 to the x-axis (see Fig. S2). Thus, the pHPZC

Fig. 2. FE-SEM images of PAMD (a) and PAMDA with 5 wt.% [Amim]Cl (b).

129

100

100

90

90

remval

80

qe

70

70

60

60

50

50

40

40

30

30

20

20 1

2

3

4

5

6

7

e

80

q (mg·g-1)

8

pH Fig. 4. Effect of pH on Cr (VI) adsorption from aqueous solution by PAMDA with 5 wt.% [Amim]Cl in term of removals (solid lines) and the equilibrium adsorption capacities (qe, dashed lines) (initial Cr (VI) concentration: 100 mg L1; T: 303 K; adsorbent dosage: 0.1 g; t = 20 min).

value of PAMDA lies at the pH0 value of 6.5. It is clear that PAMDA is negatively charged in the range of pH > 6.5 and positively charged in the range of pH < 6.5. For Cr (VI) ion, HCrO 4 and Cr2O2 7 are predominating species at the pH 2–6 and H2CrO4 is predominant at pH < 1, while in basic medium it exists as CrO2 4 [40]. When the pH value is lower than 6.5, there should be strong electrostatic attractions between positively charged surfaces of PAMDA and Cr (VI) anions, resulting in the high removal of Cr (VI) anions. In addition, at a lower pH, a large quantity of hydronium ions (H3O+) existing in solution make functional groups such as imidazole groups in the PAMDA chains more protonated. As a result, a higher removal rate of Cr (VI) anions can be achieved [41]. However, when the pH value is higher than pHPZC, the repulsive forces between the negatively charged copolymers and Cr (VI) anions enhance, therefore decreasing the adsorption capacity.

3.5. Effect of adsorbent dosage Amount of adsorbent plays a vital role in the adsorption process. Fig. 5 indicates the effect of PAMDA dosage on the removal efficiency of Cr (VI) anions and adsorption capacity of PAMDA from 50 mL of 100 mg L1 Cr (VI) solution at 303 K and pH 2, for a contact time of 20 min. As shown in Fig. 5, the removal percentage of Cr (VI) increases with the increase of PAMDA dosage from 0.02 g to 0.1 g. When the dosage of PAMDA is 0.1 g, the maximum removal efficiency of 90.6% is obtained. However, further increase in adsorbent dosage has little effect on the removal efficiency. From Fig. 5, a reverse trend of the adsorption capacity of PAMDA for Cr (VI) is 95

Contact time is an important variable during the uptake of pollutants from wastewater by adsorption. In water pollution control, the adsorption equilibrium time is an important parameter to predict the efficiency and feasibility of an adsorbent [44]. Fig. 6 shows the effect of contact time on the adsorption capacity of PAMDA at three different temperatures. The adsorption capacities of PAMDA drastically increase at the first 10 min at three different temperatures, and thereafter, remain almost unchanged. The increased adsorption capacity may be due to abundant vacant surface sites available for adsorption at the initial stage [45]. With a gradual decrease in the number of active sites and the increase of electrostatic repulsive forces between the Cr (VI) anions adsorbed on PAMDA [46], the uptake of Cr (VI) anions is slowed and finally reaches the equilibrium. The adsorption equilibrium is all reached within 10 min at these three temperatures. Cationic hydrogel can interact strongly with Cr (VI) anions via two different mechanisms: adsorption by swelling and electrostatic interaction [29]. The introduction of ionic liquid improves the hydrophilic property of the synthesized copolymer hydrogel, which enhances the swollen character of hydrogel. At the same time, the ionic conductivity of macromolecule is also improved by incorporation of positively charged [Amim]Cl. The swollen cross-linked polymeric networks with high ionic conductivity not only diminish the Cr (VI) diffusion

50

140 Removal Efficiency qe

80

120 100

-1

80

75

45

60

70

-1

85

q e (mg.g )

Cr(VI) Removal (%)

3.6. Effect of contact time

160

90

65

also observed. The adsorption capacity decreases with the increasing amount of PAMDA dosage. When PAMDA dosage is low, the active sites of adsorbent can be efficiently occupied by Cr (VI) anions, so the calculated adsorption capacity is high due to the small mass of adsorbent [42]. However, because of the low dosage of adsorbent and the constant Cr (VI) concentration, the surface areas of adsorbent may be not enough for all Cr (VI) anions to be adsorbed; so many Cr (VI) anions are left in the solution, leading to a low removal rate. Further increase of PAMDA dosage provides more surface areas and available adsorption sites, which will enhance removal of Cr (VI) but decrease the calculated adsorption capacity. When the adsorption equilibrium is reached, the increase of PAMDA dosage increases only the unoccupied surface areas in case of constant initial Cr (VI) concentration, but affects little on the total occupied surface areas. Naturally, the calculated adsorption capacity by using all the mass of adsorbent would be reduced [43], while the removal percentage keeps unchanged. Taking into consideration of the above factors, the optimum dosage of PAMDA for 50 mL of 100 mg L1 Cr (VI) solution was chosen as 0.1 g for further experiments.

q (mg·g )

Cr(VI) Removal (%)

Y. Jiang et al. / Journal of Colloid and Interface Science 455 (2015) 125–133

40 35

303K 313K 323K

30

40 0.02

0.04

0.06

0.08

0.10

0.12

Adsorbent dosage (g) Fig. 5. Effect of PAMDA dosage on Cr (VI) adsorption in term of removals (solid lines) and the equilibrium adsorption capacities (qe, dashed lines) (initial Cr (VI) concentration: 100 mg L1; T: 303 K; pH = 2; t = 20 min).

25

0

10

20

30

40

50

60

t (min) Fig. 6. Effect of contact time on the adsorption capacity of PAMDA at various temperatures (initial Cr (VI) concentration: 100 mg L1; adsorbent dosage: 0.1 g; pH = 2).

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limitation but also enhance the electrostatic interaction during the adsorption. Thus, Cr (VI) anions can easily reach to the active sites of the polymer resulting in a rapid adsorption process. The equilibrium capacities are found to be 45.30, 45.90, 47.95 mg g1, and the corresponding removal percentages of Cr (VI) are 90.6, 91.8 and 95.9% at the temperature of 303–323 K. From the data, it can be seen that the uptake of Cr (VI) slightly increases with the increase of temperature, indicating that the adsorption is controlled by an endothermic reaction. High temperature may favor the adsorption of Cr (VI) anions onto PAMDA; however, the increase of temperature does not significantly increase the adsorption capacity from the adsorption data. For its practical application, choosing an appropriate temperature for adsorption of Cr (VI) onto PAMDA with a high removal percentage at a relatively low cost is very necessary and feasible.

Cr (VI) adsorption. Pseudo-first-order and pseudo-second-order kinetic models are two widely used models to analysis the solid– liquid adsorption. The kinetic models are expressed as follows [45,47].

logðq1  qt Þ ¼ log q1 

1.5

1.2

303K 313K 323K

1.0

303K 313K 323K

t/qt (min.g.mg-1)

-1

log (q 1 -q t) (mg.g )

ð3Þ

where qt (mg g1) is the amount of adsorbed Cr (VI) on per unit mass of PAMDA at the time of t; q1 and q2 (mg g1) are the maximum adsorption capacity; k1 and k2 (g mg1 min1) are the pseudo-first-order rate constant and the pseudo-second-order rate constant. The values of q1 and k1 can be obtained from the interceptions and slopes of the respective linear plots of log(q1  qt) versus t (Fig. 7a) and q2 and k2 can be calculated by the interceptions and slopes of the linear plots of t/qt against t (Fig. 7b). The parameters of k1, k2, q1, q2, and correlation coefficients, R21 and R22 for the Cr (VI) anions adsorption onto PAMDA at the three

Kinetic studies are essential for the understanding of adsorption mechanism and evaluation of PAMDA adsorbent performance for

0.5 0.0 -0.5

a 2

0.8 0.6 0.4

b

0.2

-1.0 0

ð2Þ

t 1 1 ¼ þ t qt k2 q22 q2

3.7. kinetic studies

1.0

K1 t 2:303

4

6

8

0.0

10

0

10

20

30

40

50

t (min)

t (min) 50

q t (mg.g-1 )

45

40

303K 313K 323K

35

c

30

1

2

3

4 1/2

t

5

6

7

1/2

(min )

Fig. 7. linear plots of log(q1  qt) versus t (a), linear plots of t/qt versus t (b), and linear plots of qt versus t1/2 (c) for the Cr (VI) adsorption onto PAMDA at various temperatures.

Table 1 The pseudo-first-order and pseudo-second-order model parameters for Cr (VI) anion adsorption by PAMDA with 5 wt.% [Amim]Cl. T (K)

303 313 323

Pseudo-first-order

Pseudo-second-order

qe.exp (mg g1)

q1 (mg g1)

k1 (min1)

R21

q2 (mg g1)

k2 (g mg1 min1)

R22

45.30 45.90 47.95

31.32 57.33 55.14

0.3233 0.5435 0.5455

0.9444 0.7916 0.8480

45.31 46.51 48.66

0.0332 0.0339 0.0341

0.9993 0.9996 0.9997

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temperatures of 303–323 K are calculated from the plots in Fig. 7a and b. These parameters, correlation coefficients and the maximum adsorption capacities (qe.exp) obtained from the experiments are listed in Table 1. From Fig. 7a and b, it can be easily observed that the experimental data in Fig. 7a totally deviate from linearities; however, the plots of t/qt versus t in Fig. 7b give straight lines. The result indicates that in analyzing the Cr (VI) adsorption onto PAMDA the pseudo-second-order kinetic model is more applicable than the pseudo-first-order model. Table 1 also shows that the maximum adsorption capacities q2, determined using the pseudo-second-order plots perfectly agree with the experiment data qe.exp. Moreover, the coefficients of determination (R22) values from the plots of t/qt versus t are very high (>0.999). These results further confirm that the adsorption process of Cr (VI) anions onto PAMDA follows the pseudo-second order kinetic model rather than the pseudo-first-order model. Weber–Morris model is widely used for prediction the rate-control step of adsorption process, which can be expressed by the following equation [48]:

qt ¼ K i t1=2 þ C

ð4Þ

where Ki (mg g1 min1/2) is the diffusion rate constant and C is the thickness of the boundary layer. The values of C and Ki can be obtained from the interceptions and slopes of the linear plots of qt versus t1/2 (Fig. 7c). From Fig. 7c, it is evident that the adsorption process can be roughly divided to two steps, and adsorption via a two-step process has been previously reported in various aqueous systems [49,50]. The first sharp step can be attributed to Cr (VI) anions binding or anchoring with the positively charged active sites on PAMDA surface via electrostatic interaction. The second step represents the intra-particle diffusion, which is a very slow process due to the extremely low concentration of Cr (VI) anions in solution. So, the adsorption capacity is almost unchanged with time. The type of adsorption can be estimated by the magnitude of Arrhenius activation energy (Ea). Arrhenius activation energy (Ea) can be obtained by the following equation.

ln k2 ¼ ln A 

3.8. Effect of initial Cr (VI) concentration To determine the effect of initial Cr (VI) concentration on the removal of Cr (VI), adsorption experiments were carried out with the initial Cr (VI) concentrations ranging from 20 to 120 mg L1 at three different temperatures. Fig. 8 shows the effect of initial Cr (VI) concentration on the removal of Cr (VI) onto PAMDA. It is clear that the removal percentage of Cr (VI) decreases slowly with the increase of initial Cr (VI) concentration. The maximum removal percentages of over 97% are obtained at the 20 mg L1 Cr (VI) aqueous solutions at all three temperatures. At a lower Cr (VI) concentration, the ratios between the available adsorption sites of PAMDA surfaces and the initial number of Cr (VI) anions are relatively higher, resulting in a higher removal percentage. Subsequently, with the increase of Cr (VI) concentration, a increasing number of Cr (VI) anions will fiercely compete for the constant available adsorption sites, as a result, more Cr (VI) anions are left unabsorbed in the solution due to the saturation of binding sites. In addition, it is also interesting to note that the removal of Cr (VI) slightly increases with the increase of temperature in Fig. 8, confirming the endothermic nature of the adsorption process. 3.9. Adsorption isotherms Two well-known adsorption isotherm models, Langmuir and Freundlich isotherm models, are commonly used to describe the adsorption process. The corresponding equations of Langmuir and Freundlich isotherms are represented as follows: Langmuir equation [52]:

  1 1 1 1 ¼ þ qe qmax qmax K L C e

ð6Þ

Freundlich equation [53]:

60 50

Ea RT

ð5Þ qe(mg.g-1)

40

where Ea (kJ mol1) is the Arrhenius activation energy of adsorption; A (g mg1 min1) is the temperature-independent factor; R (J mol1 K1) is the gas constant and equal to 8.314; T (K) is the solution temperature; k2 (g mg1 min1) is the pseudo-second-order rate constant. From the slope of the straight line (Ea/R) (see Fig. S3), the activation energy Ea is calculated to be 1.094 kJ mol1, indicating this adsorption is physisorption [51].

30 303K 313K 323K Langmiur isotherm Freundlich isotherm

20 10 0

2

Removal %

6

8

10

12

14

16

Fig. 9. Comparison of experimental adsorption isotherm of Cr (VI) with adsorption isotherm models for Cr (VI) adsorption onto PAMDA at three temperatures.

90

80

Table 2 Isotherms for Cr (VI) anion adsorption by PAMDA at three different temperatures.

303K 313K 323K

70

60

4

Ce( mg.L-1)

100

20

40

60

80

100

Isotherms

Parameters

303 K

313 K

323 K

Langmuir

qmax (mg g1) KL (L mg1) R2L

66.1046 0.2396 0.9940

66.6176 0.2966 0.9900

74.5117 0.3850 0.9895

Freundlich

1/n KF (mg g1)(L mg1)1/n R2F

0.4598 15.7853 0.9816

0.4575 17.5615 0.9877

0.4664 22.2107 0.9546

120

Initial Cr(VI) concentration (mg.L-1) Fig. 8. Effect of initial Cr (VI) concentration on the removal of Cr (VI) by PAMDA at three temperatures (adsorbent dosage: 0.1 g; pH = 2; t = 20 min).

Temperature

132

Y. Jiang et al. / Journal of Colloid and Interface Science 455 (2015) 125–133

Table 3 Thermodynamic parameters for Cr (VI) anion adsorption by PAMDA.

DGo (kJ mol1) 303 K 5.708

313 K 6.286

ln qe ¼ ln K F þ

323 K 8.465

1 ln C e n

DHo (kJ mol1)

DSo (J K1 mol1)

35.8

136.173

ð7Þ

where qe (mg g1) is the equilibrium absorption amount of Cr (VI) (mg g1); Ce (mg L1) is the equilibrium concentration in solution; qmax (mg g1) is the monolayer capacity of the adsorbent; KL is the Langmuir constant related to the free energy of adsorption; KF and n are the Freundlich constant and the heterogeneity factor, respectively. The Cr (VI) adsorption data at different initial Cr (VI) concentrations at three temperatures of 303–323 K are fitted using Langmuir and Freundlich isotherms (Fig. 9), respectively. A list of the obtained adsorption constants together with R2 values at three different temperatures is provided in Table 2. It is observed that both Langmuir and Freundlich model fit well with the adsorption data (R2 > 0.95), but the isotherm data at three temperatures are better described by Langmuir model compared to Freundlich model for the relatively higher correlation coefficient (R2 > 0.9895) for Langmuir adsorption model at three temperatures than those for Freundlich adsorption model.

anionic pollutants in certain aqueous systems [29,30,55,56]. In this study, a novel ionic liquid 1-allyl-3-methylimidazolium chloride ([Amim]Cl) modified copolymer hydrogel PAMDA was successfully synthesized by a water solution copolymerization method. The as-prepared copolymer hydrogel was first used as an adsorbent to remove toxic Cr (VI) anions from aqueous solution. The obtained results confirmed that the modification of ionic liquid [Amim]Cl efficiently enhanced the adsorption capacity of PAMDA for Cr (VI) ions. Significantly, this ionic liquid functionalized hydrogel PAMDA was a rapid adsorbent compared with some reported adsorbents for Cr (VI) ion adsorption [10,11,19,20]. The adsorption equilibrium of Cr (VI) ions onto PAMDA was achieved within 10 min, which was much shorter than those of the above reported adsorbents (105 min [10]; 240 min [11]; 4 h [19] and 180 min [20]). The introduction of ionic liquid [Amim]Cl not only improved the hydrophilic property of the synthesized copolymer hydrogels, but also increased the ionic conductivity of macromolecule by incorporation of positively charged [Amim]Cl. As a result, the adsorption capacity of PAMDA was increased and the adsorption equilibrium time of Cr (VI) ions was shortened. The adsorption equilibrium well fitted with Langmuir isotherm model. The maximum adsorption capacity for Cr (VI) ions increased with the increase of temperature and was 74.5 mg L1 at 323 K based on Langmuir isotherms model. Hence, the as-prepared ionic liquid functionalized copolymer hydrogels show potential applications in wastewater treatment for the removal of Cr (VI) highly and rapidly. Acknowledgments

3.10. Thermodynamic study Thermodynamic study of an adsorption process can reveal the feasibility and spontaneous nature of the adsorption process [1]. The free energy changes (DGo, kJ mol1), standard enthalpy changes (DHo, kJ mol1) and the entropy changes (DSo, J mol1 K1) are three basic thermodynamic parameters associated with the adsorption process. These thermodynamic parameters are determined using the following equations:

DGo ¼ RT ln

ln



C a :e Ce



  C ae DSo DH o ¼  Ce R RT

ð8Þ

This research was sponsored by the National Basic Research Program of China (21175061 and 31470434), Research Foundation for Advanced Talents of Jiangsu University (10JDG142) and Special Financial Grant from the China Postdoctoral Science Foundation (2014T70488). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.05.030. References

ð9Þ

1

where Ca.e (mg L ) is the amount of Cr (VI) adsorbed onto PAMDA at equilibrium, Ce (mg L1) is the equilibrium concentration of Cr (VI) in solution, R (8.314 J mol1 K1) is the gas constant and T (K) is the solution absolute temperature. The values of DHo and DSo are calculated from the slope and interception by plotting ln(Ca.e/Ce) versus 1/T. The calculated thermodynamic parameters are showed in Table 3. The values of DGo are 5.708, 6.286 and 8.465 kJ mol1 at 303, 313 and 323 K, respectively. The negative values of DGo show the adsorption of Cr (VI) anions onto PAMDA is highly spontaneous and favorable. Moreover, the values of DGo slightly decrease with an increase of temperature, indicating the improved adsorption performance is obtained at higher temperature. The positive value of DSo (136.17 J K1 mol1) reflects increased randomness at the solid/solution interface during the adsorption. The positive DHo of 35.80 kJ mol1 indicates the endothermic nature of adsorption and the possibility of physical adsorption [54]. 4. Conclusions Hydrogels with water-swollen and cross-linked polymeric networks have been reported for the adsorption of cationic or

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