PADETA) interpenetrating polymer networks (IPNs) and its adsorption towards salicylic acid from aqueous solutions

Chemical Engineering Journal 248 (2014) 216–222

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

A novel post-cross-linked polystyrene/polyacryldiethylenetriamine (PST_pc/PADETA) interpenetrating polymer networks (IPNs) and its adsorption towards salicylic acid from aqueous solutions Jianhan Huang ⇑, Li Yang, Xiaomei Wang, Hebing Li, Limiao Chen, You-Nian Liu College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China

h i g h l i g h t s  A strategy to synthesize PST_pc/PADETA IPNs was proposed.  PST_pc/PADETA IPNs had a large equilibrium capacity towards salicylic acid.  Adsorption of salicylic acid on PST_pc/PADETA IPNs was a fast process.  PST_pc/PADETA IPNs had a large dynamic capacity towards salicylic acid.  Regeneration of PST_pc/PADETA IPNs was regenerated after the adsorption process.

a r t i c l e

i n f o

Article history: Received 25 January 2014 Received in revised form 14 March 2014 Accepted 17 March 2014 Available online 25 March 2014 Keywords: Post-cross-linking Polystyrene/polyacryldiethylenetriamine (PST/PADETA) Interpenetrating polymer networks (IPNs) Adsorption Salicylic acid

a b s t r a c t We developed an effective strategy to synthesize a novel post-cross-linked polystyrene/polyacryldiethylenetriamine (PST_pc/PADETA) interpenetrating polymer networks (IPNs) for improving the adsorption of salicylic acid from aqueous solutions. PST_pc/PADETA IPNs was synthesized from polystyrene (PST) successfully and it possessed a very large equilibrium capacity towards salicylic acid, and the equilibrium capacity of salicylic acid on PST_pc/PADETA IPNs is about double relative to the precursor PST. The Freundlich model was more appropriate for fitting the equilibrium data than the Langmuir model, and the isosteric enthalpy decreased with increasing of the fractional loading. The pseudo-second-order rate equation could characterize the kinetic data and the micropore diffusion model could describe the kinetic data very well. At an initial concentration of 1003.6 mg/L and flow rate of 11.25 BV/h, the breakthrough and saturated capacities were measured to be 82.40 and 151.0 mg/mL wet resin, respectively, and the resin column could be completely regenerated and repeatedly used by 0.01 mol/L of sodium hydroxide (w/v) aqueous solutions. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Wastewater contaminated by aromatic organic compounds such as phenol, salicylic acid and some other volatile organic compounds (VOCs) is a significant concern to the environment due to its high concentration (COD is higher than 2000 mg/L), high chroma and high toxicity [1,2]. For instance, salicylic acid is widely employed as the raw material in organic synthesis, and it is well known for its great capability to ease aches and pains, reduce fevers as an anti-inflammatory drug [3–5]. However, salicylic acid is capable of causing moderate chemical burns of the skin at a high ⇑ Corresponding author. Tel.: +86 73188879616. E-mail address: [email protected] (J. Huang). http://dx.doi.org/10.1016/j.cej.2014.03.061 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

concentration (>1.5%) and it can induce headache, nausea and even affect the liver and kidney. Especially, biological degradation of salicylic acid is not feasible due to the electron-withdrawing carboxyl group on the benzene ring [6,7]. As a result, efficient adsorptive removal and recycling of salicylic acid is of great importance and has attracted numerous attentions in recent years [8,9]. Salicylic acid has the formula of C6H4(o-OH)COOH, where the phenolic hydroxyl group is ortho to the carboxyl group. Moreover, the adjacent carboxyl group can form intramolecular hydrogen bonding with the phenolic hydroxyl group [10–12], which makes salicylic acid a well-balanced molecule of hydrophobic portion and hydrophilic portion. If polymeric adsorbent with both of hydrophilic and hydrophobic properties is utilized and applied for adsorption, and salicylic acid is used as the adsorbates, the

J. Huang et al. / Chemical Engineering Journal 248 (2014) 216–222

hydrophobic portion of salicylic acid is more inclined to approach the hydrophobic polymer networks of the hydrophilic–hydrophobic polymeric adsorbents, whereas the hydrophilic portion has a relatively strong affinity towards the hydrophilic polymer networks, and a much enhanced adsorption will be achieved accordingly. During the past three decades, interpenetrating polymer networks (IPNs) are proven one of the most efficient polymeric materials applied in reinforced rubbers, toughened plastics, damping materials, coatings and functional materials due to their unique forced compatibility [13–15]. Moreover, IPNs technology is an outstanding method for stable integration of two polymer networks with different properties or different functions by physical entanglements [16–18]. Due to the strong phase separation liability between the hydrophobic polymer networks and the hydrophilic polymer networks, at present the hydrophobicity or hydrophilicity of the two polymer networks composed of the IPNs is similar, at least not opposite, while few researches are reported for both hydrophobic and hydrophilic IPNs (hydrophobic–hydrophilic IPNs) in the literatures [19–21]. Moreover, the Brunauer–Emmett–Teller (BET) surface area of this kind of hydrophobic–hydrophilic IPNs is very low, resulting in a low adsorption of the aromatic compounds. In comparison with the IPNs technology, hyper-cross-linked technology is proven to be very effective for improving the BET surface area and pore volume of the polymeric adsorbents due to its unique reaction mechanism [22,23]. The hyper-cross-linked technology contains a Friedel–Crafts alkylation reaction of linear polystyrene or low cross-linked polystyrene with bi-functional cross-linking reagents under the help of Friedel–Crafts catalysts including anhydrous zinc chloride, iron (III) chloride and stannic (IV) chloride. The networks of the synthesized hyper-cross-linked polymeric adsorbents consist of intensive bridges of strongly solvated polystyrene chains with conformational rigid links, leading to a major shift of their pore diameter distribution from predominant mesopores to mesopores/micropores bimodal distribution, and hence results in a great increase of the BET surface area and pore volume. Because of these significant changes, the hyper-crosslinked polymeric adsorbents display very large adsorption capacities towards the aromatic compounds in aqueous solutions [24]. In addition, Jiang et al. [25], Cheng et al. [26] and Zhou et al. [27] have extensively studied the reaction kinetics of the pendant vinyl groups of divinylbenzene (DVB) in the suspension polymerization of styrene with DVB, and they found that the reaction activity of the second pendant vinyl groups of DVB was much lower than the first pendant vinyl groups. Therefore, many residual pendant vinyl groups remained on the polymer after the suspension polymerization [25–29]. In particular, these pendant vinyl groups can also be reacted with the neighboring benzene rings of the polymer according to the Friedel–Crafts alkylation reaction under the help of the Friedel–Crafts catalysts [27]. A great deal of micropores are also produced and the BET surface area of the post-cross-linked polymer is sharply increased after the reaction, inducing a great enhanced adsorption of the aromatic compounds [30–33]. Based on the discussions above, in this study, the post-crosslinked polystyrene/polyacryldiethylenetriamine (PST_pc/PADETA) IPNs were synthesized from polystyrene (PST) through interpenetration of polymethylacrylate (PMA) in the pores of PST by a typical sequential IPNs technology, post-cross-linking of the synthesized polystyrene/polymethylacrylate (PST/PMA) IPNs by a Friedel–Crafts alkylation reaction, and chemical modification of the post-crosslinked PST/PMA IPNs (named PST_pc/PMA IPNs) by an amidation reaction. After characterization of the PST_pc/PADETA IPNs, the adsorption performance of salicylic acid on this novel IPNs was investigated in comparison with PST from aqueous solutions in detail.

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2. Materials and methods 2.1. Materials and chemicals Amberlite XAD-4 beads, which were purchased from Rohm & Haas Company, were applied as the PST and the reference adsorbent in this study. Methylacrylate (MA) used as the monomer in the polymerization of PMA was washed by 5% of sodium hydroxide aqueous solutions (w/v), followed by de-ionized water, and then dried by anhydrous magnesium sulfate before using. Industrial triallylisocyanurate (TAIC) applied as the cross-linking reagent was purchased from Liuyang Chemical Co. Ltd. and its content was 98% (w/w). 2,2-Azobisisobutyronitrile (AIBN) employed as the initiator was purified by recrystallization before using. Butyl acetate, n-heptane, 1,2-dichloroethane, anhydrous ferric (III) chloride and diethylenetriamine (DETA) were also used and they were all analytical reagents. Salicylic acid employed as the adsorbate was an analytical reagent and used without further purifications.

2.2. Preparation of PST_pc/PADETA IPNs The synthetic procedure for PST_pc/PADETA IPNs was shown in Scheme S1. The PST beads were firstly swollen by a mixture of MA, TAIC, butyl acetate, n-heptane and AIBN for 24 h. MA was the polymeric monomer and TAIC was the cross-linking reagent in the suspension polymerization, and the mass ratio of MA to TAIC was defined as 9:1 and that of the monomers to PST was determined to be 1:1. Butyl acetate and n-heptane were employed as the porogens and they were 250% relative to the monomers and the mass ratio between butyl acetate and n-heptane was pre-set to be 4:1. The swollen PST beads were filtered from the mixture and added into 0.5% of polyvinyl alcohol aqueous solutions (w/w). At a moderate stirring speed, the temperature of the reaction mixture was resin to 358 K and the reaction mixture was kept at this temperature for 12 h. The resultant PST/PMA IPNs were eluted by hot deionized water for three times, extracted by petroleum ether for 24 h, and dried at 328 K for 8 h under N2 protection. According to the typical post-cross-linking method performed in Refs. [32,33], 20.0 g of PST/PMA IPNs were swollen by 100 mL of 1,2-dichloroethane overnight and 4.0 g of anhydrous ferric (III) chloride was added into the reaction mixture at 313 K. After the added ferric (III) chloride was completely dissolved, the temperature of the reaction mixture was risen to 358 K and the reaction mixture was refluxed at this temperature for 12 h. The obtained PST_pc/PMA IPNs were chemically transformed to PST_pc/PADETA IPNs via an amidation reaction of PST_pc/PMA IPNs with superfluous DETA with the temperature at 403 K for 12 h.

2.3. Characterization Fourier Transform infrared (FT-IR) spectroscopy was recorded on a Nicolet 510P Fourier transformed infrared instrument in 500–4000 cm1 with a resolution of 1.0 cm1. The BET surface area, Langmuir surface area, t-Plot surface area, pore volume t-Plot pore volume and pore size distribution were determined by N2 adsorption–desorption isotherms at 77 K using a Micromeritics Tristar 3000 surface area and porosity analyzer. The BET surface area and pore volume were calculated according to the BET model while the t-Plot surface area, t-Plot pore volume and the pore size distribution was calculated by applying the Barrett–Joyner–Halenda (BJH) method to the N2 desorption data. The weak basic exchange capacity was measured according to the method in Ref. [34].

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2.4. Bath adsorption The equilibrium adsorption of salicylic acid on the PST_pc/PADETA IPNs was performed at 288, 298, 308 and 318 K, respectively. 0.1000 g of the dry resins was accurately weighed and introduced in 50 mL of salicylic acid aqueous solutions at known initial concentrations. The initial concentrations of salicylic acid were preset to be 200.5, 401.0, 601.5, 802.0 and 1002.5 mg/L, respectively. The mixture solutions were then sealed and transferred to an incubator shaker with thermostat and shaken at a speed of 180 rpm for 12 h to ensure the adsorption process reached equilibrium. The equilibrium concentration of salicylic acid was determined and the equilibrium capacity was calculated by conducting a mass balance before and after the equilibrium. The kinetic adsorption was similar to the equilibrium adsorption except that the capacity was determined in real time until equilibrium. Dynamic adsorption was carried out with a glass column (16 mm diameter and 200 mm length) packed with 8.0 mL (1 BV, 1 BV = 8 mL) of wet resins at ambient temperature. The salicylic acid aqueous solution at an initial concentration of 1003.6 mg/L was passed through the resin column at a desired flow rate of 11.25 BV/h and the residual concentration of salicylic acid from the effluent was dynamically recorded until it reached the initial concentration. The breakthrough point (C/C0 = 0.05, where C was the concentration of salicylic acid from the effluent, mg/L) and saturated point (C/C0 = 0.95) was measured and the breakthrough and saturated capacities of salicylic acid on the resins were respectively calculated. After the dynamic adsorption, the resin column was roughly rinsed by 20 mL of de-ionized water and 0.01 mol/L of sodium hydroxide aqueous solutions (w/v) were used as the desorption solvent for the dynamic desorption. 37 BV of the desorption solvent got through the resin column at a flow rate of 5.6 BV/h and the concentration of salicylic acid from the effluent was determined until it was about zero.

3. Results and discussion 3.1. Characterization of PST_pc/PADETA IPNs As shown in Fig. 1, the FT-IR spectrum of PST/PMA IPNs is observed to be superimposed by that of PST and PMA. The strong absorption bands at 1601, 1500 and 1452 cm1 [24,35], which can be assigned to C@C stretching of the benzene ring of PST, is also appeared in the FT-IR spectrum of PST/PMA IPNs. Two strong vibrations with frequencies at 1739 and 1697 cm1 [36,37], which

are related to C@O stretching of the ester carbonyl groups of MA and the amide carbonyl groups of TAIC for PMA, are also present in the FT-IR spectrum of PST/PMA IPNs. These results reveal that not any new chemical bond is formed between PST and PMA polymer networks in addition to mutual physical entanglements between the two polymer networks. After the post-cross-linking of PST/PMA IPNs, the absorption band at 1633 cm1, which is related to C@C stretching of the pendent vinyl groups, is nearly disappeared, implying that the pendent vinyl groups of PST/PMA IPNs are completely consumed, and they are transformed to CH3CH- groups under the help of the Friedel–Crafts catalysts. Similar results were reported by Aleksieva et al. [29], Zeng et al. [32,33] and Christy et al. [38]. After the amidation of PST_pc/PMA IPNs, the strong characteristic vibration with frequency at 1739 cm1, which can be assigned to C@O stretching of the ester carbonyl groups, is sharply weakened. Whereas another new strong band appeared at 1654 cm1, and this band may be concerned with C@O stretching of the amide carbonyl groups [39,40]. Additionally, a new broad and strong vibration at 3404 cm1, which can be assigned to NAH stretching of the ANHA or ANH2 groups, is also present in the FT-IR spectrum of PST_pc/PADETA IPNs [25,40]. In particular, the weak basic exchange capacity of PST_pc/PADETA IPNs is measured to be 1.809 mmol/g (Table S1). All these results suggest the PMA polymer networks are transformed to the PADETA polymer networks successfully and the PST_pc/PADETA IPNs are synthesized accordingly. After interpenetration of PMA in the pores of PST, the BET surface area decreases from 873.1 m2/g (PST) to 563.2 m2/g (PST/PMA IPNs), and the pore volume reduces from 1.211 cm3/g to 0.8983 cm3/g (Table S1). Both of the BET surface area and pore volume increases significantly after the Friedel–Crafts alkylation reaction, similar results are reported by Ito [28] and Zeng et al. [33,34]. Moreover, 24.76 m2/g of t-Plot micropore surface area and 0.01306 cm3/g of t-Plot micropore volume are measured for PST_pc/PMA IPNs, revealing that many cross-linked bonds are produced after the post-cross-linking and obtained PST_pc/PMA IPNs contains numerous micropores. However, the amidation of PST_pc/PMA IPNs reduces the BET surface area and pore volume, which may be resulted from the increased polarity due to introduction of DETA on the surface of PST_pc/PADETA IPNs. Fig. 2 displays the pore size distribution of PST, PST/PMA IPNs, PST_pc/PMA IPNs and PST_pc/PADETA IPNs, respectively. Mesopores ranging from 5 to 15 nm are predominant for the four resins while the pore size distribution of PST_pc/PMA IPNs has a tendency to reduce as compared with PST/PMA IPNs. Actually, the average

0.025 PST PST/PMA IPNs PST_pc/PMA IPNs PST_pc/PADETA IPNs

PST Relative intensiy /(a.u.)

PST/PMA IPNs PST_pc/PMA IPNs

PST_pc/PADETA IPNs -1

-1

3404 cm 4000

-1

1601 cm

Pore volume /(cm3/g/nm)

-1

1639 cm PMA

3500

1739 cm 3000

2500

2000

0.020

0.015

0.010

0.005

-1

1654 cm 1500

1000

500

Wavenumbers /(cm-1) Fig. 1. FT-IR spectra of PST, PMA, PST/PMA IPNs, PST_pc/PMA IPNs and PST_pc/ PADETA IPNs, respectively.

0.000 0

5

10

15

20

25

30

35

40

Pore diameter /(nm) Fig. 2. Pore size distribution of PST, PST/PMA IPNs, PST_pc/PMA IPNs and PST_pc/ PADETA IPNs, respectively.

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3.2. Comparative adsorption

160 120 80 PST PST/PMA IPNs PST_pc/PMA IPNs PST_pc/PADETA IPNs 100

200

300

400

ð1Þ

Freundlich model : qe ¼ K F C 1=n e

ð2Þ

where qe and qm is the equilibrium and the maximum capacity of the adsorbates (mg/g), Ce is the equilibrium concentration of the adsorbates (mg/L), KL is a Langmuir constant (L/mg), KF ([(mg/g) (L/mg)1/n]) and n are the characteristic Freundlich constants. The equilibrium isotherm of salicylic acid on the four adsorbents are fitted by the Langmuir and Freundlich models via a non-linear fitting and the corresponding characteristic parameters qm, KL, KF, n, and the sum-square error (SSE) are summarized in Table S2. Due to the lower SSE, both of the Langmuir and Freundlich models are suitable for characterizing the equilibrium data for the adsorption on PST, PST/PMA IPNs and PST_pc/PMA IPNs, whereas only the Freundlich model is suitable for the adsorption on PST_pc/PADETA IPNs, which confirms the different adsorption mechanism of the adsorption on PST_pc/PADETA IPNs with respect to PST, PST/PMA IPNs and PST_pc/PMA IPNs. 3.3. Effect of solution pH on adsorption As shown Fig. 4, the adsorption of salicylic acid on PST_pc/PADETA IPNs is very sensitive to the solution pH. The equilibrium capacity is the largest at the original solution pH (pH = 2.61) while adding NaOH or HCl in the solution will make the adsorption weakened. According to the reported pKa1 (2.98) and pKa2 (13.1) of salicylic acid [51], the dissociation curves of salicylic acid and the mono-anion of salicylic acid are predicted as a function of the solution pH, the two curves are also displayed in Fig. 4. It is

500

600

700

800

1.0 250

pKa2 of salicylic acid

900

0.8

200 0.6

150

0.4

100

0.2

50

pKa1 of salicylic acid

0 0

0

0

K L C e qm 1 þ K LCe

300

240 _____ Langmuir model fitting _ _ _ _ Freundlich model fitting 200

40

Langmuir model : qe ¼

Equilibrium capacity /(mg/g)

Equilibrium adsorption capacity /(mg/g)

The equilibrium adsorption isotherms of salicylic acid on PST, PST/PMA IPNs, PST_pc/PMA IPNs and PST_pc/PADETA IPNs are comparatively measured, and the results are displayed in Fig. 3. The equilibrium adsorption capacity of salicylic acid on PST/PMA IPNs is rapidly decreased in comparison with PST, while that on PMA_pc/PST IPNs is relatively larger than that on PMA/PST IPNs. The BET surface area of an adsorbent plays an important role in the adsorption of the adsorbates on the adsorbent. Generally, a higher BET surface area, a larger equilibrium adsorption capacity of the adsorbates on the adsorbent is. After interpenetration of PMA in the pores of PST, the BET surface area of PST/PMA IPNs is sharply decreased in comparison with PST, while that of PST_pc/ PMA IPNs increases significantly after the post-cross-linking, and hence the above observation is observed. In addition, the equilibrium adsorption capacity of salicylic acid on PST_pc/PADETA IPNs is greatly increased in comparison with PST_pc/PMA IPNs, and it is the largest among the four considered adsorbents in the present study, which implies that the amidation of PST_pc/PMA IPNs highlights the increased equilibrium capacity. After introduction of DETA on the surface of PST_pc/PADETA IPNs, the BET surface area decreases from 645.0 (PST_pc/PMA IPNs) to 459.0 m2/g (PST_pc/PADETA IPNs), and the BET surface area of PST_pc/PADETA IPNs is much lower than that of PST. While the equilibrium capacity of salicylic acid on PST is 62.8 mg/g (according to the Freundlich fitting data), about half of that on PST_pc/ PADETA IPNs (127.4 mg/g according to the Freundlich fitting data), indicative of different adsorption mechanism. In addition to the BET surface area, the polarity matching between the adsorbent and the adsorbates also plays a significant role [34,37,39]. Salicylic acid has the hydrophilic carboxyl and phenolic hydroxyl groups. Furthermore, the adjacent carboxyl group forms intramolecular hydrogen bonding with the phenolic hydroxyl group [10–12], leading salicylic acid to be a well-balanced molecule with both of hydrophobic portion and hydrophilic portion. The benzene ring of salicylic acid is highly hydrophobic, the same as the newly formed hexatomic ring between the carboxyl group and the phenolic hydroxyl group [12]. However, the exocyclic the hydroxyl group of carboxyl group is highly hydrophilic. The adsorbent PST_pc/PADETA IPNs applied in this study possesses the

hydrophobic PST and the hydrophilic PADETA polymer networks. Therefore, as PST_pc/PADETA IPNs is employed for adsorption of salicylic acid from aqueous solutions, the particular superiority of the amphiphilic property of PST_pc/PADETA IPNs towards salicylic acid is perfectly embodied. The hydrophobic PST polymer networks of PST_pc/PADETA IPNs have a relatively strong affinity towards the hydrophobic portion of salicylic acid due to the hydrophobic interaction or p–p stacking [41–44], while the hydrophilic PADETA polymer networks are more inclined to approach the hydrophilic portion of salicylic acid by the possible static interaction and hydrogen bonding [45–48], and consequently achieving a highly efficient adsorption (Scheme S2). Langmuir and Freundlich models are frequently adopted to describe the equilibrium process [49,50]. They can be given as:

2

4

6

8

0.0 10

12

14

Proportion of the molecular state /(%)

pore diameter of PST/PMA IPNs is 6.38 nm while that of PST_pc/ PMA IPNs is 6.23 nm, which confirms that plenty of micropores are produced for PST_pc/PMA IPNs.

Initial solution pH

Equilibrium concentration /(mg/L) Fig. 3. Equilibrium isotherms of salicylic acid on PST, PST/PMA IPNs, PST_pc/PMA IPNs and PST_pc/PADETA IPNs from aqueous solutions with the temperature at 298 K.

Fig. 4. Solution pH effect on the adsorption of salicylic acid on PST_pc/PADETA IPNs from aqueous solutions at 298 K (for salicylic acid, pKa1 = 2.98, pKa2 = 13.1, and the dissociation curves of salicylic acid were predicted on dependency of the solution pH).

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interesting to observe that the adsorption has the same tendency as the dissociation curve of salicylic acid as the solution pH is ranged from 2.61 to 10.30, implied that the molecular form of salicylic acid is favorable for the adsorption. As the solution pH increases from 10.30 to 12.43, the equilibrium capacity further decreases and it is nearly zero as pH = 12.43, this tendency is also similar to the dissociation curve of the mono-anion of salicylic acid, and which suggested that the mono-anionic state of salicylic acid can be adsorbed, while the di-anion of salicylic acid cannot be adsorbed. On the other hand, the adsorption is also weakened as the solution pH is in the range of 2.61–0.69, which might be resulted from the uploaded amino groups on the surface of PST_pc/PADETA IPNs [52].

Fig. 6 displays the kinetic curves for adsorption of salicylic acid on PST_pc/PADETA IPNs at 298, 308 and 318 K, respectively. The adsorption capacity increases rapidly with increasing of adsorption time, and it reaches 52% relative to the equilibrium capacity within one hour at 298 K and reaches 73% at 318 K, implying a faster process at a higher temperature. All of the adsorption can reach equilibrium within 350 min, suggesting that the adsorption is a fast process. Lagergren pseudo-first-order and pseudo-second-order rate equations were applied for fitting the kinetic data [53,54]. They can be arranged as:

3.4. Equilibrium adsorption

Lagergren pseudo-first-order : qt ¼ qe ð1  ek1t Þ

ð5Þ

Pseudo-second-order : qt ¼ qe k2 t=ð1 þ qe k2 tÞ

ð6Þ

Fig. 5 shows the equilibrium isotherms of salicylic acid on PST_pc/PADETA IPNs at 288, 298, 308 and 318 K, respectively. The temperature is favorable for the adsorption and a higher temperature induces a greater adsorption, indicative of an endothermic process. Table S3 summarizes the parameters fitted by the Langmuir and Freundlich models, and which indicated that the experimental data are consistent with the fitting curves based on the Freundlich model and the KF increased with increasing of the temperature. Following the Clausius–Clapeyron equation:

d ln C e DH ¼ 2 dT RT

ð3Þ

where DH (kJ/mol) is the isosteric enthalpy at a given equilibrium capacity or a fractional loading (h, where h ¼ q0e =qe ; q0e is the given adsorption capacity and qe is the plotted equilibrium adsorption capacity according to the Langmuir isotherm equation), T is the temperature (K) and R is the gas constant (8.314 J/(mol K)). Eq. (4) can be followed by integration of Eq. (3):

ln C e ¼ DH=ðRTÞ þ C 0

ð4Þ

where C0 is the integral constant. As plotting ln Ce versus 1/T, all of the isosters (ln Ce versus 1/T) are straight lines and DH can be calculated from the slopes of the straight lines. As displayed in Fig. S1, the DH is positive, implying the endothermic character of the adsorption. Moreover, the DH decreased with increasing of the fractional loading, suggesting that PST_pc/PADETA IPNs possesses surface energy heterogeneity [40,42].

3.5. Kinetic adsorption

here qt (mg/g) is the capacity at a given time t (min) and k1 (min1) and k2 (g/(mg min)) is the pseudo-first-order and pseudo-secondorder rate constants. Table S4 summarizes the corresponding parameters by the two kinetic models. It can be concluded that only the pseudo-secondorder rate equation is suitable for characterizing the kinetic data, similar results were reported for adsorption of salicylic acid on some other adsorbents [8,9]. Additionally, the k2 at 298, 308 and 318 K is predicted to be 8.342  105, 1.620  104 and 2.258  104 g/(mg min), which are in agreement with the observed phenomenon above. A micro-pore diffusion model is also employed to analyze the kinetic data. According to Ruthven et al. [55], as the fractional adsorption uptake (qt/qe) is less than 85%, the adsorption kinetics in micro-porous adsorbents can be described by the following equation as:

qt 6 ¼ pffiffiffiffi qe p

sffiffiffiffiffiffiffi Dc t 3Dc t  2 r 2c rc

ð7Þ

where Dc is the micropore diffusivity (cm2/s) and rc is the crystal diameter of the adsorbent (cm). The kinetic data in Fig. 6 are fitted by Eq. (7) and the simulated results are shown in Fig. S2. It is observed that the kinetic data could be well characterized by the micropore diffusion model as the fractional adsorption uptake is less than 85%, confirming that a great many micropores are existent for PST_pc/PADETA IPNs.

250

180

225

160

200 140 120

150

qt /(mg/g)

qe /(mg/g)

175

288 K 298 K 308 K 318 K ____ Langmuir model fitting _ _ _ Freundlich model fitting

125 100 75 50

100 80

298 K 308 K 318 K ______Pseudo-first-order rate equation fitting _ _ _ _Pseudo-second-order rate equation fitting

60 40

25

20

0 0

100

200

300

400

500

600

700

Ce /(mg/L) Fig. 5. Equilibrium isotherms of salicylic acid on PST_pc/PADETA IPNs from aqueous solutions with the temperature at 288, 298, 308 and 318 K, respectively (the Langmuir and Freundlich models are applied for simulation of the experimental isotherm data).

0

0

50

100 150 200 250 300 350 400 450 500 550

t /(min) Fig. 6. Kinetic curves of adsorption of salicylic acid on PST_pc/PADETA IPNs from aqueous solutions at 298, 308 and 318 K, respectively (the pseudo-first-order and pseudo-second-order rate equations are applied for the simulation of the experimental kinetic data).

J. Huang et al. / Chemical Engineering Journal 248 (2014) 216–222

3.6. Dynamic adsorption and desorption The dynamic adsorption and desorption property of salicylic acid on PST_pc/PADETA IPNs is investigated and the results are depicted in Fig. 7(a) and (b), respectively. Fig. 7(a) shows that the shape of the dynamic adsorption curve is very sharp, indicating that the adsorption of salicylic acid on PST_pc/PADETA IPNs reaches equilibrium quickly after leakage. We defined C/C0 = 0.05 as the breakthrough point and C/C0 = 0.95 as saturated point, and the volume of the effluent to reach the breakthrough point was defined as Vb and that to the saturated point was Vs. Fig. 7(a) displays that the Vb and Vc is 82.1 BV and 150.5 BV, and the corresponding breakthrough capacity was calculated to be 82.40 and 151.0 mg/ mL wet resin, respectively, confirming that PST_pc/PADETA IPNs is truly an efficient polymer for adsorptive removal of salicylic acid from aqueous solutions. After the dynamic adsorption, different desorption solvents are employed for the desorption process. The recovery efficiency of different solvents for desorption of salicylic acid from the resin column was displayed in Fig. S4. It is evident that water can hardly desorb salicylic acid and only 14.67% of salicylic acid is desorbed from the resin column. Meanwhile, sodium hydroxide aqueous solutions are very effective for desorption and increasing the concentration of sodium hydroxide leads a higher recovery efficiency. Almost 100% of salicylic acid can be recovered as 0.01 mol/L of sodium hydroxide aqueous solutions (w/v) are utilized and hence we used this solution as the 12 BV of the desorption solvent is enough for complete regeneration of the resin column and the dynamic

(a)

1.0

desorption capacity was calculated to be 892.1 mg (Fig. 7(b)), which was excellently coincident with the dynamic saturated capacity (895.2 mg). The PST_pc/PADETA IPNs were used repeatedly for five cycles in the continuous adsorption–desorption process and they exhibited good reusability with remarkable regeneration behaviors (Fig. S5). 4. Conclusions PST_pc/PADETA IPNs were prepared, characterized and evaluated for adsorptive removal of salicylic acid from aqueous solutions. PST_pc/PADETA IPNs possesses a very large equilibrium capacity and it reached 127.4 mg/g at an equilibrium concentration of 100 mg/L. The Freundlich model was more appropriate for fitting the equilibrium data than the Langmuir model and the isosteric capacity decreased with increasing of the adsorption loading. The pseudo-second-order rate equation was more suitable for characterizing the kinetic data than the pseudo-first-order rate equation and kinetic data could be excellently simulated by the micropore diffusion model. The breakthrough and saturated capacities were measured to be 82.40 and 151.0 mg/mL wet resin at an initial concentration of 1003.6 mg/L and a flow rate of 11.25 BV/h, and the resin column could be completely regenerated by 0.01 mol/L of sodium hydroxide aqueous solutions (w/v). Acknowledgments The authors are gratefully appreciated to the National Natural Science Foundation of China (Nos. 21174163 and 21376275) and the Shenghua Yuying Project of Central South University for the financial supports. Appendix A. Supplementary material

0.8 Adsorbed capacity: 895.2 mg

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.03.061.

0.6

C/C0

221

0.4

References 0.2 0.0 0

25

50

75

100

125

150

175

Concentration of salicylic acid /(mg/L)

Volume of the effluent /(BV, 1BV=8 mL) 30000

(b)

25000 Desorbed capacity: 892.1 mg

20000 15000 10000 5000 0 0

5

10

15

20

25

Volume of the effluent /(BV, 1BV=8 mL) Fig. 7. Dynamic adsorption (a) and desorption (b) curves of salicylic acid on PST_pc/ PADETA IPNs resin column from aqueous solutions (For the adsorption: 8.0 mL of wet resins, C0 = 1003.6 mg/L, flow rate = 11.25 BV/h. For the desorption: desorption solvent = 0.01 mol/L of sodium hydroxide aqueous solutions (w/v), flow rate = 5.6 BV/h).

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