Polar-modified post-cross-linked polystyrene and its adsorption towards salicylic acid from aqueous solution

Chemical Engineering Journal 286 (2016) 400–407

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Polar-modified post-cross-linked polystyrene and its adsorption towards salicylic acid from aqueous solution Xiao Ling a, Hebing Li b, Hongwei Zha b, Chunlian He a,⇑, Jianhan Huang b,⇑ a b

College of Medicine, Hunan Normal University, Changsha, Hunan 410081, China College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China

h i g h l i g h t s
An effective approach for improving the polarity of post-cross-linked polystyrene was developed.
A novel polar-modified post-cross-linked polystyrene PGDpc_D was prepared.
PGDpc_D possessed a much enhanced adsorption in comparison with its precursors and PDVBpc.
The adsorption was a fast process and the micropore diffusion model fitted the kinetic data.
PGDpc_D was a potential candidate for treatment of salicylic acid from aqueous solution.

a r t i c l e

i n f o

Article history: Received 21 September 2015 Received in revised form 4 November 2015 Accepted 5 November 2015 Available online 11 November 2015 Keywords: Post-cross-linked polystyrene Polarity Adsorption Salicylic acid

a b s t r a c t A novel polar-modified post-cross-linked polystyrene PGDpc_D was prepared by the Friedel–Crafts alkylation reaction of the pendent vinyl groups and amination reaction with diethylenetriamine (DETA). The Brunauer–Emmett–Teller (BET) surface area and pore volume of the starting copolymer PGD increased significantly after the post-cross-linking, and the surface polarity of the post-cross-linked polystyrene PGDpc improved greatly after the amination reaction. Batch adsorption runs of salicylic acid on PGDpc_D were studied using its precursors (PGD and PGDpc) and the non-polar post-cross-linked polystyrene PDVBpc as the references. Experimental results indicated that PGDpc_D possessed a much enhanced adsorption towards salicylic acid in comparison with PGD, PGDpc and PDVBpc, and the equilibrium data could be characterized by both of the Langmuir and Freundlich models. The adsorption was a fast process and the kinetic data obeyed the micropore diffusion model. Column adsorption–desorption experiments suggested that PGDpc_D was a potential candidate for treatment of salicylic acid from aqueous solution. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction In 1970s, Davankov et al. [1] synthesized a kind of novel polymeric adsorbent from linear polystyrene by the Friedel–Crafts alkylation reaction, and this novel polymeric adsorbent is generally called ‘‘hyper-cross-linked polystyrene”. Hyper-cross-linked polystyrene possesses high Brunauer–Emmett–Teller (BET) surface area, adjustable pore structure and excellent recycling property [2,3], and hence is extensively applied as the column packing materials in high-performance liquid chromatography (HPLC), ion size-exclusion chromatography and solid-phase extraction for gases, organic contaminants and organic vapors [4–9]. However, ⇑ Corresponding authors. E-mail addresses: [email protected] (C. He), [email protected] (J. Huang). http://dx.doi.org/10.1016/j.cej.2015.11.014 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

the synthetic procedure for the hyper-cross-linked polystyrene is frequently confronted with some thorny problems, and the most serious one is using chloromethyl methylether (CMME) as the post-cross-linking reagent in the Friedel–Crafts alkylation reaction because CMME is recognized as the strong carcinogen. Hence, many researchers are exploiting new methods for synthesis of hyper-cross-linked polystyrene without using CMME. Macintyre et al. [10] and Bratkowska et al. [11] used vinylbenzyl chloride (VBC) as the polymer monomer containing benzyl chloride and the obtained hyper-cross-linked polystyrene also exhibits high BET surface area and excellent adsorption properties. However, much difference is existent for the monomer reactivity ratio between styrene and VBC, and the production cost of VBC is also rather high. In 1988, Ando et al. [12] developed post-cross-linked polystyrene by consuming the self residual pendent vinyl groups of high cross-linking polystyrene without adding external

X. Ling et al. / Chemical Engineering Journal 286 (2016) 400–407

cross-linking reagent. A considerable number of pendent vinyl groups are proven to be located in the dense cores for the high cross-linking polystyrene [13], and they can also be further cross-linked with the benzene rings of the neighboring polymeric chains by the Friedel–Crafts alkylation reaction, resulting in an increased BET surface area and preferable pore structure [14–17]. As a result, the post-cross-linked polystyrene has attracted many attentions in recent years [18–23]. Due to its extremely hydrophobic surface, the post-cross-linked polystyrene had a relatively low adsorption to polar aromatic compounds [18,19]. To improve the surface polarity of the post-crosslinked polystyrene and increase the equilibrium capacity, Zeng et al. [20–22] synthesized a series of polar-modified post-crosslinked polystyrene by introducing polar monomers such as methyl acrylate (MA), vinyl pyridine (VP) and ethylene glycol dimethacrylate (EGDMA) in the polymerization, the prepared polar-modified post-cross-linked polystyrene possessed an enhanced adsorption due to the increased polarity. Ma et al. [23] prepared another polar-modified post-cross-linked polystyrene using methyl methacrylate (MMA) as the polar monomer, and a synergistic effect is shown for adsorption of tetracycline and Cu2+. Nevertheless, the existing studies indicate that the polarity increasing of the polar-modified post-cross-linked polystyrene is limited, and the enhanced adsorption of the considered resins is not obvious. Amino and amide groups are proven the most efficient polar groups for greatly improving the surface polarity of the resins [24–26], and the increased polarity will induce a much enhanced adsorption via electrostatic interaction or hydrogen bonding [25– 29]. We proposed that if some specific amino, amide and hydroxyl groups were uploaded on the surface of the post-cross-linked polystyrene by a specific chemical reaction, the polarity increasing of the considered resin will be obvious, leading to a much enhanced adsorption. In particular, we focused on improving the surface polarity of the post-cross-linked polystyrene, and used the obtained polar-modified post-cross-linked polystyrene for adsorption of salicylic acid from aqueous solution. For this purpose, glycidyl methacrylate (GMA) was adopted as the polar monomer and poly(glycidylmethacrylate-co-divinylbenzene) (PGD) was prepared by a typical suspension polymerization. GMA, as an efficient polymer monomer in the polymerization, contains both vinyl and epoxy groups. The vinyl groups of GMA allow it copolymerization functionality with some other polymer monomers like DVB, and the epoxy groups permit its structural modification of the polymer backbone that can result in differentiated properties and higher performance. The Friedel–Crafts alkylation reaction was then carried out for the starting copolymer PGD, the residual pendent vinyl groups of PGD were further cross-linked and the post-cross-linked polystyrene PGDpc was synthesized. After that, an amination reaction was executed for PGDpc and hence the polar-modified postcross-linked polystyrene PGDpc_D was prepared. Salicylic acid was selected as the adsorbate to evaluate the adsorption of PGDpc_D using its precursors (PGD and PGDpc) and the nonpolar post-cross-linked polystyrene PDVBpc as the references.

2. Experimental 2.1. Materials GMA and divinylbenzene (DVB, purity: 80%) were purchased from Gray West Chengdu Chemical Co. Ltd., they were washed by 5% of NaOH (w/v) and followed by de-ionized water, and then dried by anhydrous magnesium sulfate before use. Benzoyl peroxide (BPO) employed as the initiator was recrystallized by methanol. Toluene, n-heptane, 1,2-dichloroethane (DCE), anhydrous ferric (III) chloride, and diethylenetriamine (DETA) were obtained from

401

Yongda Chemical Co., and they were all analytical reagents. Salicylic acid employed as the adsorbate was used without further purification. 2.2. Preparation of the polar-modified post-cross-linked polystyrene The preparation of the spherical non-polar post-cross-linked polystyrene PDVBpc was performed according to the method in Refs. [18,30]. The polar-modified post-cross-linked polystyrene PGDpc_D was prepared by the given procedure in Scheme 1. The starting copolymer PGD was synthesized by the usual suspension polymerization using GMA as the polar monomer and it was 5% or 10% relative to the monomers (w/w), and DVB was the crosslinking reagent. Toluene and n-heptane were used as the porogens, they were 200% relative to the monomers (w/w), and the mass ratio between toluene and n-heptane was defined as 4:1. The reaction mixture was polymerized at 358 K for 12 h and the prepared spherical PGD beads were collected, washed and extracted by petroleum ether in Soxhlet apparatus for 12 h. The Friedel–Crafts alkylation reaction was then carried out using DCE as the solvent and anhydrous ferric (III) chloride as the Friedel–Crafts catalyst [30]. The reaction temperature was kept at 358 K and the reaction time was 10 h. The obtained post-cross-linked polystyrene PGDpc was extracted by ethanol for 12 h, and it was chemically transformed to the polar-modified post-cross-linked polystyrene PGDpc_D (or PGDpc_D_10% as GMA was 10% relative to the total mass of the monomers) by an amination reaction with superfluous DETA at 393 K for 12 h [31]. 2.3. Characterization of the polar-modified post-cross-linked polystyrene Fourier transform infrared spectra (FT-IR) of the resins were recorded on a Nicolet 510P Fourier transform infrared instrument in 500–4000 cm1 with a resolution of 1.0 cm1. The pore structure of the resins was determined by N2 adsorption–desorption isotherms at 77 K using a Micromeritics Tristar 3000 surface area and porosity analyzer. The weak basic exchange capacity of the resins was measured according to the back titration method described in Ref. [32]. The hydroxyl groups of the resins were determined by the acylation method in Ref. [33]. The concentration of salicylic acid in aqueous solution was analyzed via a 2450 UV spectrophotometer at the maximum wavelength of 296.5 nm. 2.4. Equilibrium and kinetic adsorption For the equilibrium adsorption, about 0.1 g of the resins were mixed with 50 mL of a series of salicylic acid aqueous solutions. The initial concentrations of salicylic acid were set to be about 200, 400, 600, 800 and 1000 mg/L, respectively. The series of solutions were shaken in a thermostatic oscillator at three different temperatures (293, 303 and 313 K, respectively) until the equilibrium was reached. The equilibrium concentration of salicylic acid Ce (mg/L) was determined and the equilibrium capacity qe (mg/g) was calculated based on the following equation:

qe ¼ ðC 0  C e Þ  V=W

ð1Þ

where C0 was the initial concentration of salicylic acid (mg/L), V was the volume of the solution (L) and W was the mass of the resins (g). The kinetic adsorption of the resins was similar to the equilibrium adsorption except that the adsorption capacity was determined in real time until equilibrium. During this process, 0.5 mL of the solution was sampled at different time intervals, and the concentration of the residual salicylic acid solution was measured until the equilibrium was reached.

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CH

CH=CH2

O

CH2

CH

Polymerization

CH COCH2CHCH2

CH2=CH COCH2CHCH2 +

O

O

CH

CH=CH2

GMA CH

CH

CH2 O

DCE

CH CH2

CH2

DVB CH2

CH2 O

CH

CH COCH2CHCH2

PGD CH2

DETA

CH

CH2 O

OH

CH COCH2CHCH2

O

FeCl3

NHCH2CH2NHCH2CH2NH2

CH CH2

CH

CH

CH3

CH CH2

CH3

PGDpc

PGDpc_D

Scheme 1. Synthetic procedure of the polar-modified post-cross-linked polystyrene PGDpc_D.

2.5. Column adsorption and desorption 10 mL of the wetted resins were densely packed in a glass column with an inner diameter of 16 mm to set up a resin column, and this resin column was used for the column adsorption experiment. The initial concentration of salicylic acid aqueous solution was 995.8 mg/L, the temperature of resin column was 293 K and it kept constant during the column adsorption. Salicylic acid aqueous solution was passed through the resin column at a flow rate of 1.4 mL/min, and the concentration of salicylic acid from the effluent was recorded continuously until it reached the initial concentration. After the column adsorption, the desorption solvent containing 0.01 mol/L of NaOH (w/v) and 20% of ethanol (v/v) was passed through the resin column at a flow rate of 0.8 mL/min, the concentration of salicylic acid in the effluent was determined until it was close to zero. 3. Results and discussion 3.1. Characterization of the polar-modified post-cross-linked polystyrene As shown in Fig. 1, a very strong absorption band at 1730 cm1 presented in the FT-IR spectrum of PGD, and this band can be assigned to the C@O stretching of the ester carbonyl groups of GMA [34,35]. Additionally, two characteristic bands at 1630 and 996 cm1 were also displayed in the FT-IR spectrum, and they are related to the residual pendent vinyl groups (ACH@CH2) of

-1

1682 cm

Relative intensity /(a.u.)

PDVB

-1

1630 cm

PDVBpc PGD

Table 1 Structural parameters of PGD, PGDpc and PGDpc_D as well as PDVB and PDVBpc.

PGDpc PGDpc_D

4000

3500

2

-1

-1

1730 cm

3640-3270 cm

3000

2500

2000

DVB. Furthermore, they are also emerged in the FT-IR spectrum of the non-polar post-cross-linked polystyrene PDVB, Aleksieva et al. [15], Zhou et al. [16], Zeng et al. [22] and Zhou et al. [30] reported the similar results. After the Friedel–Crafts alkylation reaction, the bands related to the ACH@CH2 groups were almost disappeared, demonstrating that the residual pendent vinyl groups of the starting copolymers are consumed after the post-crosslinking. This phenomenon is in good agreement with some other literatures [20,21]. The amination reaction made the vibration at 1730 cm1 almost disappear, while another band at 1682 cm1 appeared in the FT-IR spectrum of PGDpc_D. The band at 1682 cm1 is concerned with the C@O stretching of the amide carbonyl groups [24,28]. Li et al. got the similar results [35–37]. In addition, a broad vibration appeared in the range of 3640– 3270 cm1 [38–40], the weak basic exchange capacity of PGDpc_D was measured to be 1.58 mmol/g, and the hydroxyl groups was determined to be 0.35 mmol/g. All of these results confirm that amino, amide and hydroxyl groups are uploaded on the surface of PGDpc_D successfully. Table 1 indicated that the BET surface area and pore volume of PGD were a little less than those of PDVB, which may be due to the introduction of GMA in the copolymer. After the Friedel–Crafts alkylation reaction, there was an evident increase of the BET surface area, and the increased value was 430.4 m2/g for PDVBpc, while a much less one was measured for PGDpc (207.1 m2/g), implying that the residual pendent vinyl groups of PGD were much less than PDVB. Zeng et al. [22] found that the concentration of MMA in the polymerization exerted a strong influence on the pendent vinyl groups of the copolymer, and the residual vinyl groups decreased to zero as MMA was up to 40 mol%. In particular, the t-plot micropore surface area of PGDpc increased sharply from 5.028 m2/g to 66.25 m2/g, and the pore volume as well as the t-plot micropore volume had the same trend. These results imply that quantities of methylene cross-linking bridges are formed in the dense pores of PGD and many micropores are produced after the post-cross-linking. The amination induced a decreased BET surface area due to the increased polarity [39,40].

1500

1000

500

-1

Wavenumbers /(cm ) Fig. 1. FT-IR spectra of PGD, PGDpc and PGDpc_D as well as PDVB and PDVBpc.

BET surface area/(m /g) t-Plot microporous surface area/(m2/g) Pore volume/(cm3/g) t-Plot micropore volume/(cm3/g) Diameter/(mm)

PDVB

PDVBpc PGD

PGDpc

PGDpc_D

658.4 24.54

1088.9 121.2

791.2 66.25

693.8 45.54

584.1 5.028

1.625 2.095 1.510 1.740 1.820 0.01287 0.07747 0.002443 0.02804 0.01977 0.1–0.3

0.1–0.3

0.1–0.3

0.1–0.3

0.1–0.3

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X. Ling et al. / Chemical Engineering Journal 286 (2016) 400–407

160

(a)

140 120

qe /(mg/g)

Fig. 2 is the pore diameter distribution of PGD, PGDpc and PGDpc_D, and Fig. S1 shows the corresponding pore diameter distribution of PDVB and PDVBpc. Mesopores in the range of 3–20 nm are the predominant pores for PGD and PDVB, while PGDpc and PDVBpc present more micropores after the post-cross-linking, and the newly formed micropores consist of another micro/mesopore region (2–5 nm). Additionally, the pore volume of PGDpc is rapidly decreased after the amination, while the pore diameter distribution does not change obviously. Fig. S2 shows the scanning electron micrograph (SEM) images of PGDpc_D, and it is seen that PGDpc_D are spherical polymeric beads with abundant pores on the surface.

100 80 60 PDVBpc PGDpc_D

40 20 0

3.2. Equilibrium adsorption

0.04 PGD PGDpc PGDpc_D

3

dV/dD /(cm /g)

0.03

0.02

0.01

0.00

0

10

20

30

40

50

60

70

80

90

Pore diameter /(nm) Fig. 2. Pore diameter distribution of PGD, PGDpc and PGDpc_D.

100

100 200 300 400 500 600 700 800 900 1000 Ce /(mg/L)

160

(b)

140 120 qe /(mg/g)

Fig. 3(a) compared the equilibrium isotherms on PGDpc_D and PDVBpc at 293 K, and salicylic acid was used as the adsorbate. It is found that PGDpc_D exhibits a much larger equilibrium capacity than PDVBpc. At an equilibrium concentration of 100 mg/L, the equilibrium capacity of salicylic acid on PGDpc_D was measured to be 83.58 mg/g, while only 39.39 mg/g of equilibrium capacity was determined for PDVBpc. PDVBpc has a much higher BET surface area than PGDpc_D, but the much lower equilibrium capacity of PDVBpc in comparison with PGDpc_D, demonstrating that some other factors play important roles on the adsorption. Hydrophobic interaction, electrostatic interaction and hydrogen bonding are commonly considered the important factors influencing the adsorption [18,41,42]. The surface of PGDpc_D contains considerable polar groups such as amino and hydroxyl groups, and the polarity matching between PGDpc_D and salicylic acid may account for the increased equilibrium capacity [17,18,29]. Fig. 3(b) compared the equilibrium isotherms of the starting copolymer PGD, the post-cross-linked polystyrene PGDpc and the polar-modified post-cross-linked polystyrene PGDpc_D. It is clear that PGDpc has a larger equilibrium capacity than PGD, which may be resulted from the increased BET surface area due to the post-cross-linking. After the post-cross-linking, more p–p sites are available for PGDpc, which are helpful for the adsorption [31]. Moreover, after the surface modification, PGDpc_D has an even larger equilibrium capacity in comparison with PGDpc. The BET surface area and pore volume of PGDpc_D were less than PGDpc, while the amino, amide and hydroxyl groups enhance the adsorption due to the polarity matching, similar results are shown in Refs. [37–40]. Additionally, Fig. 3(b) indicated that PGDpc_D_10% (GMA was 10% relative to the total mass of the monomers (w/w)) had a

0

100 80 PGD PGDpc PGDpc_D PGDpc_D_10%

60 40 20 0

0

100 200 300 400 500 600 700 800 900 1000 Ce /(mg/L)

Fig. 3. Equilibrium isotherms of salicylic acid on (a) PDVBpc and PGDpc_D; (b) PGD, PGDpc and PGDpc_D from aqueous solution at 293 K.

relatively smaller equilibrium capacity than PGDpc_D (GMA was 5%), suggesting that a suitable mass ratio of the polar monomer in the polymerization makes an enhanced adsorption. Similar results are listed in Refs. [20,22]. We further increased the mass ratio of GMA to 20% and synthesized another polar-modified post-cross-linked resin PGDpc_D_20 (GMA was 20% relative to the total mass of the monomers (w/w)). The equilibrium capacity of salicylic acid on PGDpc_D_20% was shown to be smaller than that on PGDpc_D_10%, and much smaller than that on PGDpc_D. In particular, PGDpc_D_20% had a much smaller equilibrium capacity towards salicylic acid as compared with that on the non-polar post-cross-linked resin PDVBpc, which suggested a much weakened adsorption with increasing of the mass ratio of the polar monomer in the polymerization. As compared PGDpc_D with some other adsorbents for considering the adsorption of salicylic acid, it is found that PGDpc_D is inferior to the activated charcoal [43], while the activated charcoal cannot be repeatedly used, PGDpc_D can be repeatedly used (shown in the subsequent section). In addition, PGDpc_D is superior to some low-cost materials such as bentonite, hematite and calcite [44,45], and comparable to some other polymeric adsorbents such as the post-cross-linked polystyrene PST_pc/PADETA (a post-crosslinked polystyrene/polyacryldiethylenetriamine) [17]. XAD-4 [31], XAD-7 [31], the hyper-cross-linked polystyrene HJ-L02 (a bisphenol-A-modified hyper-cross-linked resin) [46] and HJ-M05 (a diethylenetriamine-modified hyper-cross-linked resin) [31].

X. Ling et al. / Chemical Engineering Journal 286 (2016) 400–407

Fig. 4 gives the equilibrium isotherms of salicylic acid adsorption on PGDpc_D from aqueous solution with the temperature at 293, 303 and 313 K, respectively, and Fig. S3 shows the corresponding equilibrium isotherms of salicylic acid adsorption on PDVBpc. It is seen that the adsorption is weakened with increasing of the temperature, and the equilibrium capacities of salicylic acid on PDVBpc and PGDpc_D at 313 K were relatively smaller than the corresponding ones at 293 K, implying an exothermic process [32,47,48]. Langmuir and Freundlich models [49,50] were adopted to describe the equilibrium data via a non-linear fitting and the corresponding characteristic parameters such as qm, KL, KF and n as well as the correlation coefficients R2 are summarized in Table S1. Both of the Langmuir and Freundlich models are suitable for fitting the equilibrium data since R2 > 0.98. With increasing of the temperature, the KL, KF and qm decrease, indicating that the adsorption at a higher temperature is less effective [48]. According to the Clausius–Clapeyron equation [5,13]:

d ln C e DH ¼ 2 dT RT

ð2Þ

where DH is the isosteric adsorption enthalpy (kJ/mol). If DH is negative, the adsorption is exothermic, that is the overall decrease in enthalpy is achieved by the generation of heat. On the other hand, if DH is positive, the adsorption is endothermic, that is heat is absorbed by the system after the adsorption. As DH has nothing to do with the temperature or it changes little with the difference of the temperature, and Eq. (2) will be followed by integral method as:

40 PDVBpc PDGpc_D

30

20

10

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

The fractional loading Fig. 5. Isosteric adsorption enthalpies of salicylic acid absorbed on PDVBpc and PGDpc_D as a function of the fractional loading (h).

and hydroxyl groups are uploaded on PGDpc_D, which make it highly heterogeneous and the adsorption involves a chemical bond formation between the adsorbate and the adsorbent [31,46]. 3.3. Effect of the solution pH on adsorption

180

As shown in Fig. S4, the adsorption of salicylic acid on PDVBpc was very sensitive to the solution pH. The original solution pH of salicylic acid aqueous solution was measured to be 3.08, the adsorption at this pH was the strongest among all the pH used in this study, and the equilibrium capacity was measured to be 152.7 mg/g. As adding sodium hydroxide in the original salicylic acid aqueous solution, the pH of the solution increases, while the equilibrium capacity sharply decreases. The equilibrium capacity of salicylic acid on PDVBpc was measured to be 10.2 mg/g at a solution pH of 12.4, which is only about 6% (w/w) relative to the corresponding one at the original pH of salicylic acid aqueous solution (pH = 3.08). This observation implies that the molecular form of salicylic acid is favorable for the adsorption while the anionic state of salicylic acid is unfavorable for the adsorption. In addition, as adding hydrochloric acid in the salicylic acid aqueous solution, the initial solution pH decreases, it is interesting to observe that there presents slight decrease for the equilibrium capacity, and the equilibrium capacity decreases to 135.6 mg/g at a solution pH of 1.05, which may be from the uploaded polar functional groups on the surface of PGDpc_D.

150

3.4. Kinetic adsorption

DH ln C e ¼  þ C0 RT

ð3Þ

where C0 is the integral constant. As plotting ln Ce vs. 1/T at a given equilibrium capacity, all the isosters (ln Ce vs. 1/T) can be fitted to straight lines and DH can be calculated from the slopes of the straight lines using Eq. (3). Fig. 5 displays the plotting of DH as a function of the fractional loading (h) (h = qe0 /qmax, qe0 is the given equilibrium capacity and qmax is the maximum capacity). It is clear that DH is negative, indicating an endothermic process [32,48]. Furthermore, it is interesting to see that the DH on PGDpc_D decreases strongly with increasing of the fractional loading while that on PDVBpc keeps almost constant, implying a different surface character of these two adsorbents. PDVBpc is non-polar post-cross-linked polystyrene and it holds a homogeneous surface, inducing a general physical adsorption. Many polar groups such as amino, amide

120

qe /(mg/g)

50

H /(kJ/mol)

404

90 293 K 303 K 313 K Freundlich Fitting Langmuir Fitting

60 30 0

0

100

200

300

400

500

600

700

800

900

Ce /(mg/L) Fig. 4. Equilibrium isotherms of salicylic acid on PGDpc_D at 293, 303 and 313 K, respectively.

The kinetic adsorption of PGDpc_D was comparatively studied using its precursors PGD, PGDpc and the non-polar post-crosslinked polystyrene PDVBpc as the references and the results are plotted in Fig. 6. The capacity increases rapidly with increasing of the adsorption time and it reaches over 90% within one hour, implying that the adsorption is a fast process. The pseudo-firstorder and pseudo-second-order rate equations [51,52] were applied for analyzing the kinetic data and the he corresponding parameters are listed in Table S2. It can be deduced that both of the pseudo-first-order and pseudo-second-order rate equations are appropriate for characterizing the kinetic data due to R2 > 0.98 and the fact that the calculated qm are close to the experimental ones. Meanwhile, the k2 values on the post-cross-linked polystyrene are shown to be less than the corresponding the starting copolymers. The k2 values on PDVBpc and PGDpc are predicted

405

100

1.0

80

0.8

40

PDVB PDVBpc PGD PGDpc PGDpc_D

20 0

(a)

0.6

60

C/C0

qt /(mg/g)

X. Ling et al. / Chemical Engineering Journal 286 (2016) 400–407

0

20

40

60

80

100

0.4 PDVBpc PDG PDGpc PDGpc_D

0.2 0.0

120

0

140

10

20

30

t /(min)

40

50

60

70

80

90 100 110

Volume of the effluents /(BV)

3

3

to be 6.011  10 and 3.358  10 g/(mg min), while those of PDVB and PGD are scaled to be 6.293  103 and 4.221  103 g/ (mg min), respectively. This fact may be attributed to their different pore structure. Compared with the starting copolymers, the post-cross-linked polystyrene has more micropores. The diffusion resistance of salicylic acid in the micropores is greater than that in the macropores and mesopores, and hence the apparent adsorption rate on PDVBpc and PGDpc_D is lower. Many micropores are formed after the post-cross-linking and hence the kinetic adsorption of the post-cross-linked polystyrene is quite different from the starting copolymers, and hence the kinetic data were further dealt by a micropore diffusion model. According to Ruthven et al. [53], as the fractional adsorption uptake (qt/qe) was less than 85%, the kinetic data in microporous adsorbents could be described as:

sffiffiffiffiffiffiffi qt 6 Dc t 3Dc t  2 ¼ pffiffiffiffi rc qe p r2c

ð4Þ

where Dc is the micropore diffusivity (cm2/s) and rc is the resin diameter (cm). The adsorption capacities less than 85% of the equilibrium were fitted by the micropore diffusion model, the correlated parameters Dc/rc2 and the correlation coefficients are illustrated in Table S3. Table S3 indicates that the micropore diffusion model characterizes the kinetic data on PDVBpc, PGDpc and PGDpc_D due to R2 > 0.98, while it is not suitable for the starting copolymers, confirming that many micropores are produced for PDVBpc and PGDpc after the post-cross-linking.

Concentration of salicylic acid /(mg/L)

Fig. 6. Kinetic curves of salicylic acid on PGD, PGDpc and PGDpc_D as well as PDVB and PDVBpc at 293 K and the initial concentration at 600.3 mg/L.

(b)

64000 56000 48000 40000

PGDpc_D PDVBpc

32000 24000 16000 8000 0 0

2

4

6

8

10

12

14

16

18

20

Volume of the effluents /(BV) Fig. 7. Dynamic (a) adsorption curves of salicylic acid on PGD, PGDpc and PGDpc_D as well as PDVB and PDVBpc resin column; (b) desorption curves of salicylic acid from the PGDpc_D and PDVBpc resin column.

that of PGD, PGDpc and PDVBpc (35.9, 41.0 and 37.5 BV, respectively). The dynamic capacity was calculated using a numerical integration of the breakthrough curves by Eq. (5), and the corresponding breakthrough and saturated capacities can be calculated to be 31.82 and 71.40 mg/mL wet resin.

Z qdynamic ðmg=gÞ ¼ 0

BV max

   mg Cv dBV  C 0 1 L C0

10 mL 1ðLÞ   2:770 g 1000ðmLÞ

ð5Þ

3.5. Column adsorption and desorption The equilibrium and kinetic adsorption of salicylic acid on PGDpc_D is satisfactory, it is hopeful that PGDpc_D can be developed as an efficient polymeric adsorbent for adsorptive removal of salicylic acid from aqueous solution. Hence the dynamic adsorption and desorption of PGDpc_D was investigated and the results are shown in Fig. 7(a) and (b). Fig. 7 states that the dynamic adsorption curves from the leakage point to the saturated point are very sharp, indicating that the adsorption is a fast process and it reaches equilibrium quickly after leakage. Here C/C0 = 0.05 is defined as the breakthrough point (C is the concentration of salicylic acid from the effluent, mg/L), the volume of the effluent to reach the breakthrough point is defined as Vb. Fig. 7(a) indicates that Vb of PGDpc_D is measured to be 59.4 BV, much higher than

After the column adsorption, the resin column was roughly rinsed by 10 mL of de-ionized water, the desorption solvent including 0.01 mol/L of NaOH (w/v) and 20% of ethanol (v/v) was used for the desorption of salicylic acid from the resin column (Fig. 7(b)). At a flow rate of 0.8 mL/min, only 8 BV of the desorption solvent is enough to completely regenerate the resin column, and the dynamic desorption capacity was 701.8 mg for PGDpc_D (493.6 mg for PDVBpc), very close to the saturated capacity (714.0 mg for PGDpc_D and 508.2 mg for PDVBpc, respectively) in the column adsorption. Continuous adsorption-regeneration runs of PGDpc_D were performed for five cycles (Fig. S5), and PGDpc_D exhibited good reusability with remarkable regeneration behaviors.

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4. Conclusions We developed a novel method for improving the surface polarity of the post-cross-linked polystyrene and increasing the equilibrium capacity to salicylic acid from aqueous solution. The polar-modified post-cross-linked polystyrene PGDpc_D was synthesized from PGD by the Friedel–Crafts alkylation reaction of the pendent vinyl groups and amination reaction with DETA. Both of the BET surface area and pore volume of the post-cross-linked polystyrene PGDpc increased significantly after the post-crosslinking and the polarity increased greatly after the amination reaction. PGDpc had a larger equilibrium capacity to salicylic acid than PGD due to the higher BET surface area, and PGDpc_D possessed a much larger one than PGDpc because of the increased polarity. The equilibrium data were well fitted by both of the Langmuir and Freundlich models, the micropore diffusion model characterized the kinetic data. Column adsorption results showed that the breakthrough and saturated capacities were 31.82 and 71.40 mg/mL wet resin, and the spent PGDpc_D could be easily regenerated by 8 BV of 0.01 mol/L of NaOH (w/v) and 20% of ethanol (v/v). Acknowledgments The authors are gratefully acknowledged the National Natural Science Foundation of China (Nos. 21174163, 21176063, 21376275 and 21446016) and South Wisdom Valley Innovative Research Team Program for the financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.11.014. References [1] S.V. Rogozhin, V.A. Davankov, M.P. Tsyurupa, Patent USSR 299165, 1969. [2] V.A. Davankov, M.P. Tsyurupa, Hypercrosslinked polymeric networks and adsorbing materials, Compr. Anal. Chem. 56 (2011) 1–648. [3] M.P. Tsyurupa, T.A. Mrachkovskaya, L.A. Maslova, G.I. Timofeeva, L.V. Dubrovina, E.F. Titova, V.A. Davankov, V.M. Menshov, Soluble intramolecularly hypercrosslinked polystyrene, React. Polym. 19 (1993) 55–66. [4] L.D. Belyakova, T.I. Schevchenko, V.A. Davankov, M.P. Tsyurupa, Sorption of vapors of various substances by hypercrosslinked ‘‘styrosorb” polystyrenes, Adv. Colloid Interf. Sci. 25 (1986) 249–266. [5] G.I. Rosenberg, A.S. Shabaeva, V.S. Moryakov, T.G. Musin, M.P. Tsyurupa, V.A. Davankov, Sorption properties of hypercrosslinked polystyrene sorbents, React. Funct. Polym. 1 (1983) 175–182. [6] M.P. Tsyurupa, V.A. Davankov, Hypercrosslinked polymers: basic principle of preparing the new class of polymeric materials, React. Funct. Polym. 53 (2002) 193–203. [7] M.P. Tsyurupa, V.A. Davankov, Porous structure of hypercrosslinked polystyrene: state-of-the-art mini-review, React. Funct. Polym. 66 (2006) 768–779. [8] V.A. Davankov, M.P. Tsyurupa, Structure and properties of hypercrosslinked polystyrene-the first representative of a new class of polymer networks, React. Polym. 13 (1990) 27–42. [9] A. Tadim, V.A. Davankov, M.P. Tsyurupa, Structure and properties of porous hypercrosslinked polystyrene sorbents ‘‘styrosorb”, Pure Appl. Chem. 61 (1989) 1881–1888. [10] F.S. Macintyre, D.C. Sherrington, L. Tetley, Synthesis of ultrahigh surface area monodisperse porous polymer nanospheres, Macromolecules 39 (2006) 5381– 5384. [11] D. Bratkowska, N. Fontanals, F. Borrull, P.A.G. Cormack, D.C. Sherrington, R.M. Marcéa, Hydrophilic hypercrosslinked polymeric sorbents for the solid-phase extraction of polar contaminants from water, J. Chromatogr. A 1217 (2010) 3238–3243. [12] K. Ando, T. Ito, H. Teshima, H. Kusano, M. Streat, Ion exchange for industry, Ellis Horwood Ltd., 1988. [13] K.L. Hubbard, J.A. Finch, G.D. Darling, The preparation and characteristics of poly (divinylbenzene-co-ethylvinylbenzene), including Ambeflite XAD-4. Styrenic resins with pendent vinylbenzene groups, React. Funct. Polym. 36 (1998) 17–30. [14] M.C. Zhang, Q. Zhou, A.M. Li, C.D. Shuang, W. Wang, M.Q. Wang, A magnetic sorbent for the efficient and rapid extraction of organic micropollutants from large-volume environmental water samples, J. Chromatogr. A 1316 (2013) 44–52.

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