Acetylaniline modified hypercrosslinked polystyrene resins and their equilibria and kinetics towards glabridin from Glycyrrhiza glabra L. extracts

Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 484–491

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Acetylaniline modified hypercrosslinked polystyrene resins and their equilibria and kinetics towards glabridin from Glycyrrhiza glabra L. extracts Xiaoting Li a,b , Yi Liu a,c , Yongfeng Liu a,c , Duolong Di a,c,∗ a Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, People’s Republic of China b University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, People’s Republic of China c Centre of Resource Chemical and New Material, 36 Jinshui Road, Qingdao 266100, People’s Republic of China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Novel acetylaniline modified hyper-

• •

• •

crosslinked polystyrene resins XT8 were synthesized and employed to adsorb glabridin. The microporous surface areas of XT8 occupied more than 55% of the total BET surface areas. The glabridin uptakes on XT8-8 were remarkably larger than that on its precursor and the commercial adsorbents like BMKB-1. The reliable regeneration performance of XT8-8 proved its practical applicability. The remarkable adsorption behaviors of XT8-8 were due to its suitable structural design and modification.

a r t i c l e

i n f o

Article history: Received 6 May 2016 Received in revised form 14 September 2016 Accepted 16 September 2016 Available online 17 September 2016 Keywords: Hypercrosslinked resin Glabridin Adsorption Equilibrium Kinetics

a b s t r a c t As reported in our previous work, crosslinked polymeric adsorbents with high microporous surface areas are proper for glabridin adsorption. In addition, adsorption capacity of the microporous and mesoporous self-crosslinked polystyrene resin XT2-10 was superior to that of the optimal commercial adsorbent BMKB-1 under the same conditions. However, there is still room for further improvement. Based on this, in the following study, a series of novel hypercrosslinked polymeric adsorbents modified with acetylaniline as the cross-linked bridge (labeled as XT8-0, XT8-4, XT8-6, XT8-8 and XT8-10) were synthesized from macroporous crosslinked chloromethylated polystyrene by adding different quantity of acetylaniline in the Friedel–Crafts reaction. The microporous surface areas of hypercrosslinked polymeric resins XT8 occupied more than 55% of the total BET surface areas and these resins were evaluated for adsorption of glabridin. Among the synthesized five resins, XT8-8 possessed the largest adsorption capacity toward glabridin, and it was superior to the previous synthesized self-crosslinked polystyrene resin XT2-10 and much better than the

∗ Corresponding author at: Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, People’s Republic of China. E-mail address: [email protected] (D. Di). http://dx.doi.org/10.1016/j.colsurfa.2016.09.067 0927-7757/© 2016 Elsevier B.V. All rights reserved.

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optimal commercial resin BMKB-1. The isotherms could be fitted by Freundlich model and the adsorption was shown to be an exothermic process. The kinetic curves could be characterized by pseudo-secondorder rate equation and the adsorption rate of glabridin on XT8-8 was controlled by both intraparticle diffusion and external diffusion. The enthalpy H, Gibb’s free energy G and entropy S were calculated to be negative. The reusability of the modified resins was also assessed and the modified resins exhibited considerable reusability. The remarkable adsorption behaviors of XT8-8 were due to its suitable structural design and modification. The hypercrosslinked resins being developed were promising alternatives to commercial adsorbents for adsorbing glabridin and other flavones from herbal plants. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Licorice, the root of Glycyrrhiza glabra L., is one of the most extensively used herbal medicines and natural sweeteners in ancient Span, Persia, India and Russia. In traditional Chinese medicine, licorice is one of the most frequently used ingredients. Licorice is promoted as an herb that can treat peptic ulcers, eczema, skin infections, cold sores, menopausal symptoms, liver disease, respiratory ailments, chronic fatigue syndrome, acquired immune deficiency syndrome (AIDS), and even cancer [1]. In particular, glabridin in licorice is a polyphenolic flavonoid and a main constituent in the hydrophobic fraction of licorice extract [2]. It has been revealed that glabridin exhibits a wide range of biological properties, such as cytotoxic activity, antimicrobial activity, and anti-proliferative and estrogenic activity against human breast cancer cells [3]. It also affects the regulation of energy metabolism, anti-nephritic, anti-tumorigenic, antibacterial and skin-whitening activities [4]. Adsorption has been known to human beings since a long time ago and it is proved to be one of the most efficient methods for separation or purification of organic compounds from aqueous solution. The preparation of porous films or resins is focused by many researchers and they have been widely applied in the field of adsorption [5–8]. In addition, many researchers have reported that the commercial macroporous adsorption resins (MARs) exhibit outstanding adsorption properties and have advantages in enriching and separating natural products [9,10]. However, the adsorption mechanisms of commercial MARs are mainly based on hydrophobic forces, such as the van der Waals force and hydrogen-bond interaction, which may lead to poor adsorption capacity and selectivity [11]. Recently, hypercrosslinked polymeric adsorbent (HCPs) has achieved increasing attention in the fields of adsorption and separation and it is recognized by its predominant micropores, relatively high uptakes, fairly good selectivity, diverse chemical structure and easy regeneration [12–14]. Many scientists have reported that the hypercrosslinked polymeric adsorbent has superior adsorption to aromatic compounds than the usual macroporous polymeric adsorbent due to the micropore filling mechanism [15]. A set of novel hypercrosslinked resin was successfully synthesized from a linear polystyrene (PS) or a low crosslinked PS by adding bi-functional cross-linking reagents such as monochloromethylether, p-dibenzenylchloride and p-dichlomethylbenzene, and Friedel–Crafts catalysts including stannic (IV) chloride, iron (III) chloride, and anhydrous zinc chloride through the Friedel–Crafts reaction in the 1970s [16–18]. After reaction, intensive networks with long-chain bridges were formed accordingly, and the pore diameter distribution had a great transfer from the macroporous region to the micro/mesoporous one which lead to a sharp increase of the Brunauer–Emmet–Teller (BET) surface area and pore volume [19,20]. In order to obtain better adsorption selectivity for a specific compound, chemical modification of HCPs is often adopted by introducing some special functional groups onto the adsorbent matrix [21,22]. In the present study, a series of hypercrosslinked

PS resins (XT8) were synthesized from macroporous crosslinked chloromethylated PS through the Friedel–Crafts reaction by adding a different quantity of acetylaniline in the Friedel–Crafts reaction. In addition, the pore structures, chemical structures and adsorption performances of the prepared resins towards glabridin are compared. The most promising resin denoted as XT8-8 was selected for detailed experimental studies for the adsorption of glabridin. The adsorption equilibrium and kinetics of glabridin on the adsorbent XT8-8 were thereafter measured and analyzed in detail. 2. Experimental 2.1. Materials and reagents Macroporous crosslinked chloromethylated PS beads used as the precursor in the Friedel–Crafts reaction was supplied by Xi’an Sunresin Technology Co., Ltd. (Shaanxi,China). Its crosslinking degree was 6%, chlorine content was measured to be 17.3%, BET surface area was determined to be 24.69 m2 /g, pore volume was 0.05 cm3 /g and average pore diameter was 16.2 nm. Anhydrous iron (III) chloride, 1,2-dichloroethane, acetonitrile, hydrochloric acid, sodium hydroxide, and ethanol was purchased from Tianjin Chemical Reagent Co., Inc. (Tianjin, China). Acetic acid was purchased from Shandong Yuwang Industrial Co., Ltd. (Shandong, China) and distilled water used was obtained in our laboratory. All solutions prepared for HPLC were subjected to filtration through 0.45 ␮m nylon membranes before use. Glabridin extract with 90% purity used as the adsorbate in this study was purchased from Nanjing Zelang Medical Technology Co., Ltd (Nanjing, China). 2.2. Synthesis of XT8 series hypercrosslinked PS resins As reported in many papers [23–26], nitrobenzene was usually used as solvent in the synthesis of hypercrosslinked polymeric adsorbent. However, nitrobenzene may present high risks to human health and ecological system because of its strong carcinogenicity [27]. Therefore, in this study, we used 1,2-dichloroethane to replace it as the solvent in the production of hypercrosslinked polymeric adsorbent. The preparation procedure for the acetylaniline modified hypercrosslinked PS resins was performed according to the Friedel–Crafts reaction and the detailed synthetic process is shown in Fig. 1. 20 g of chloromethylated PS beads were swollen by 175 mL of dichloroethane in a flask overnight at 298 K. The mass percentage of acetylaniline in the reaction was set to be 0%, 4%, 6%, 8% and 10% relative to the mass of the macroporous crosslinked chloromethylated PS (w/w), respectively. Anhydrous iron (III) chloride (2.5 g) applied as the catalysts was added into the reaction flask as quickly as possible. After the added ferric chloride was completely dissolved, the mixture was evenly heated to 353 K within 55 min using linear temperature program with gradients of 1 ◦ C/min. After keeping the reaction mixture for about 12 h, the XT8 series acetylaniline modified hypercrosslinked PS resins named as

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Fig. 1. The synthetic procedure of the XT8 series hypercrosslinked PS resins.

XT8-0, XT8-4, XT8-6, XT8-8 and XT8-10 were synthesized accordingly. To remove residual dichloroethane and ferric chloride after the reaction, the mixture was poured into an ethanol bath containing 1% of hydrochloric acid (w/w). The polymer beads were filtered, washed with deionized water until neutral pH, then extracted with ethanol in Soxhlet apparatus for 10 h and finally dried under vacuum at 323 K for 6 h. The dried sample of the resin named as XT8 was stored in airtight containers. 2.3. Characterization of adsorbents Physical properties of the resins were determined by nitrogen adsorption–desorption isotherms at 77 K using a Micromeritics ASAP 2020 surface area and porosity analyzer (Micromeritics Instrument Corp., Norcross, USA). The specific surface area was measured by the BET method while the pore volume and average pore diameter were obtained by the Barrett–Joyner–Halenda (BJH) method. FTIR spectra of resins were obtained on a FTIR spectrophotometer (Thermo Nicolet, NEXUS, USA) in the 4000–500 cm−1 region using the KBr pellet method. Chlorine content determination was carried out by an electrochemistry method, which was named as Volhard method [28–30]: the accuracy of the weight of the synthetic beads was 0.2000 g and the sample was heatdigested in a nickel crucible at 873 K for 8 h with 0.5 g of melting sodium hydroxide. The melted masses were diluted in distilled water to 250 mL, and then the electric potential of the sample aqueous solution was determined using an electrochemical workstation (RST 5200, Zhengzhou, China). The content of the chloromethyl group was calculated from the following equation with a range of 0.01–100 mmol g−1 and R2 = 0.9997. C = exp N=

 M − 15.171  53.431

0.25 × C × 1000 0.2

(1) (2)

where C (mol L−1 ) refers to the concentrations of the sample aqueous solution, M (mV) is the electric potential of the sample aqueous solution, N is the content of chloromethyl group (mmol g−1 dry adsorbent). 2.4. Batch adsorption experiment The equilibrium adsorption isotherm was performed at three different temperatures 288, 298, and 308 K, respectively. Pretreated resin (equal to 1.0 g dry resin) was mixed with 100 mL of glabridin solution with different initial concentrations in a 100 mL of conical flask and placed in a thermostatic oscillator (120 rpm) for 12 h so that the adsorption reached equilibrium. The adsorption kinetics was conducted at 288 K, 303, and 318 K, separately. An aliquot of supernatant was obtained at preset time intervals to analyze the residual concentration by HPLC. The equilibrium

Fig. 2. Quantity adsorbed of N2 on the XT8 series hypercrosslinked PS resins in function of the reduced pressure.

concentration of glabridin, Ce (mg/L), was determined and the equilibrium adsorption capacity qe (mg/g) was calculated by conducting a mass balance on glabridin before and after the adsorption experiment via the following equation: qe = (C o − Ce ) ×

V0 W

(3)

where qe is the adsorption capacity (mg/g dry resin) toward glabridin at adsorption equilibrium; C0 and Ce are the initial and equilibrium concentrations of the glabridin solutions (mg/mL), respectively; V0 is the adsorption solution volume (mL); and W is the dry weight of the resins (g). 3. Results and discussion 3.1. Characteristics of polymeric adsorbents Some important characteristics of XT8 series hypercrosslinked PS resins are listed in Table 1. When the hypercrosslinked resins are synthesized by Friedel–Crafts reaction, many new pores are formed through the cross-linked bridge and benzene ring (Fig. 1). Compared with the data of chloromethylated PS in Section 2.1, it’s easy to see that both the specific surface area and the pore volume of starting precursor chloromethylated PS increased significantly after postcrosslinking modification. In addition, microporous surface areas of hypercrosslinked polymeric adsorbents XT8 occupied more than 55% of the total BET surface area. The N2 adsorption–desorption isotherms at 77 K of XT8 series resins are demonstrated in Fig. 2. It is observed that the initial part of the adsorption isotherms of XT8 at lower relative pressure (P/P0 ) below 0.05, where the nitrogen uptake increases sharply with the

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Table 1 The characteristic parameters for the acetylaniline modified hypercrosslinked resins.

BET surface area (m2 /g) Langmuir surface area (m2 /g) t-Plot micropore surface area (m2 /g) Pore volume (cm3 /g) t-Plot micropore volume (cm3 /g) Average pore width (nm) Particle size (nm) Chlorine content (mmol/g)

XT8-0

XT8-4

XT8-6

XT8-8

XT8-10

681.6 912.8 382.7 0.56 0.17 3.58 0.3–1.2 1.98

714.5 956.2 415.6 0.58 0.19 3.49 0.3–1.2 1.86

805.8 1077.1 465.0 0.65 0.21 3.36 0.3–1.2 1.39

905.2 1211.1 518.6 0.73 0.24 3.27 0.3–1.2 1.16

1009.3 1349.9 576.0 0.78 0.26 3.19 0.3–1.2 0.96

Fig. 3. Pore diameter distribution of the XT8 series hypercrosslinked PS resins.

Fig. 4. FT-IR spectra of the chloromethylated PS, XT8-0 and XT8-8 hypercrosslinked PS resins.

3.2. Comparison of adsorption capacity of glabridin on different polymer adsorbents increment of relative pressure, proves the existence of abundant micropores. As shown in Fig. 2, the N2 uptakes of the XT8 resins was much larger than that of the chloromethylated PS [31] at the same relative pressure and those of the newly synthesized adsorbents (XT8) are type IV with obvious hysteresis loop suggesting the existence of mesoporous structures. The pore size distributions of adsorbents calculated by applying the density functional theory are shown in Fig. 3. The results showed that XT8 series resins possessed bimodal pore size distributions. In other words, XT8 series resins are typical of micropore adsorbent with a small amount of meso/macropores (2–100 nm), while the pore size of chloromethylated PS [31] is mainly distributed in the regions of meso/macropores (10–100 nm). Fig. 4 shows the IR spectra of starting precursor chloromethylated PS [31] and postcrosslinked resin XT8-0 and XT8-8. As can be seen from Fig. 4, most of the vibrations of the resins are similar. In addition, two representative vibrations with frequencies at 1260 and 670 cm−1 are very strong for the chloromethylated PS. These two vibrations may be assigned to the C Cl stretching of the CH2 Cl groups and are greatly weakened after the Friedel–Crafts reaction, in agreement with the results of the chlorine content in previous section. While a moderate vibration appears at 1705 cm−1 for XT8-0 and this vibration may be assigned to the oxidation of benzyl chloride by oxygen in the reaction system [32]. In particular, by increasing the quantity of acetylaniline, the intensity of the C O stretching becomes weakened while another strong vibration with frequencies at 3100–3700 cm−1 is presented, which is concerned with the characteristic adsorption peaks of the N H functional group, suggesting that acetylaniline has been attached to the framework of XT8.

The five acetylaniline modified hypercrosslinked PS resins possesses different pore structure (different BET surface area and pore volume) and chemical structure (different uploading amounts of acetylaniline on the surface), and hence they should exhibit different adsorption selectivity to glabridin. As shown in Table 1, the BET surface area of the five resins were measured to be 681.6, 714.5, 805.8, 905.2, 1009.3 m2 /g, and the pore volume were 0.56, 0.58, 0.65, 0.73, 0.78 cm3 /g, respectively. However, as can be seen from Fig. 5, the equilibrium adsorption capacity of glabridin on XT8-8 is the largest among the five resins. After uploading acetylaniline on the surface of the resins, the polarity and the BET surface area of the obtained resin increases while the average pore diameter decreases, so it is likely that combinations of the appropriate pore structure (BET surface area and average pore diameter) and the proper uploading amount of acetylaniline make XT8-8 a better adsorbent for glabridin. XT8-8 is hence employed as a specific polymeric adsorbent for adsorption of glabridin in the present study. 3.3. Equilibrium adsorption isotherms The adsorption isotherm is the functional expression of the relationship between the equilibrium concentration of an adsorbate in solution and the adsorption capacity of solid adsorbent at a given temperature under adsorption equilibrium conditions. Fig. 6 displays the glabridin adsorption isotherms on XT8-8 with the temperature at 288, 298 and 308 K, respectively. The glabridin uptakes increase with increasing of the equilibrium concentration and decrease with increment of the temperature, implying that the adsorption is an exothermic process.

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X. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 484–491 Table 3 Thermodynamics parameters for the adsorption of glabridin onto XT8-8. T (K)

−G (kJ/mol)

−H (kJ/mol)

−S (J/mol K)

R2

288 298 308

18.55 17.81 17.42

34.91

56.81

0.9996

Fig. 5. Effect of the added amount of acetylaniline in the Friedel–Crafts reaction on the equilibrium adsorption capacity of glabridin on the XT8 series hypercrosslinked PS resins.

Fig. 7. Adsorption kinetic curves of glabridin adsorbed on the XT8-8 hypercrosslinked PS resin with the temperature at 288, 303 and 318 K, respectively.

heterogeneous the surface was. The value of 1/n below 1 demonstrated normal isotherm, and above 1 was indicative of cooperative adsorption [36]. In the present work, the values of 1/n were in the range of 0–1, which indicated a normal isotherm. 3.4. Adsorption thermodynamics The adsorption enthalpy H (kJ/mol), adsorption Gibbs free energy G (kJ/mol) and adsorption entropy S (J/mol K) for the present adsorption system can be calculated as [37,38]:

Fig. 6. Equilibrium adsorption isotherms of glabridin adsorbed on the XT8-8 hypercrosslinked PS resin with the temperature at 288, 298 and 308 K, respectively. Table 2 Langmuir and Freundlich isotherm parameters of glabridin onto XT8-8. T (K)

288 298 308

Langmuir isotherm model

Freundlich isotherm model

qm (mg/g)

KL (L/g)

R2

KF [(mg/g)(L/mg)1/n ]

n

R2

61.16 73.21 71.63

0.14 0.30 0.36

0.9844 0.9543 0.9593

77.50 71.47 70.26

2.11 1.56 1.50

0.9941 0.9762 0.9724

In order to understand the mechanism of adsorption of glabridin on XT8-8, Langmuir [33] and Freundlich [34] isotherm equations have been employed to explain the process of adsorption equilibrium. The corresponding parameters of the two isotherm models were calculated and are tabulated in Table 2. It was found that the Freundlich model was more appropriate for describing the experimental data, which actually should be based on the curves of surface/interafce tensions to the critical micelle formation concentration (CMC) [35] and indicated that the adsorption behaviors were multilayer adsorption. According to Freundlich isotherm theory, the value of 1/n was a measure of adsorption intensity or surface heterogeneity, and the closer the value got to zero, the more

K = M/KL

(4)

G = −RT lnK

(5)

lnK = −H/RT +S/R

(6)

where R is the universal gas constant, 8.314 J/(mol K), T is the absolute temperature and K is a constant. The enthalpy values H of glabridin onto the resin XT8-8 is negative (see Table 3), implying that the adsorption of glabridin is exothermic. Meanwhile, the absolute value of the enthalpy change for XT8-8 resin was less than 43 kJ/mol, indicating that the adsorption process of glabridin on XT8-8 was governed by physical mechanism rather than chemical mechanism. In addition, the negative G indicates that the adsorption is a favorable process and the negative S suggests that the adsorption system is more ordered after the adsorption. 3.5. Adsorption kinetics It is well known that thermodynamic data can only predict the final state of a system from an initial non-equilibrium mode, and that from the perspective of engineering application, a kinetic analysis is the most important fundamental information required to determine the residence time for completion of the adsorption process. Fig. 7 depicts the influence of time on the uptake of glabridin onto the resin XT8-8 at different temperatures. The adsorption is shown to be very fast in the first 2 h (up to 70%) and then slower and

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Table 4 Kinetic parameters for adsorption of glabridin onto XT8-8. T (K)

288 303 318

Pseudo-first-order model

Pseudo-second-order model

k1 (1/min)

qe (mg/g)

R2

k2 (g/mg min)

qe (mg/g)

R2

0.0123 0.0088 0.0082

90.95 55.46 52.99

0.8329 0.8989 0.8898

0.0002 0.00018 0.00016

50.33 49.63 48.90

0.9966 0.9956 0.9964

Table 5 Intraparticle diffusion model parameters for the adsorption of glabridin on XT8-8 resin. T (K)

C1 kid,1 (mg/g min1/2 )

C2 kid,2 (mg/g min1/2 )

C3 kid,3 (mg/g min1/2 )

288 303 318

2.53 2.23 2.10

1.22 1.35 1.33

0.01 0.06 0.15

1.34 0.77 0.07

16.64 11.57 10.21

44.19 41.05 37.26

at last reach equilibrium within 600 min, implying that these adsorbents display an excellent kinetic property, which can be attributed to their excellent pore structures. As known to all, the average pore diameter and the pore distribution of the adsorbent determine the diffusion speed of the adsorbates from the solution phase to the adsorption site of the adsorbent. And it can be seen from Ref. [31] that the required time from the beginning to the equilibrium for XT2-10 (the average pore diameter of which is 3.24 nm) is about 600 min at the initial glabridin concentration of 0.45 mg/mL, longer than 540 min for XT8-8 at the same concentration. This is reasonable that the larger average pore diameter of XT8-8 (3.27 nm) decreases the diffusion resistance of glabridin, which causes the diffusion rate to increase for XT8-8. As shown in Fig. 7, the adsorption capacity of glabridin on XT8-8 is slightly smaller at a higher temperature, which is in accordant with the equilibrium adsorption results. In this study, two kinetic models, including pseudo-first-order [39] and pseudo-second-order [40], were used to investigate the adsorption process of glabridin onto the polymeric adsorbent XT88. The pseudo-first-order model is generally applicable over the initial stage of an adsorption process, while the pseudo-secondorder model assumes that the rate-limiting step is chemisorption and predicts the behavior over the entire process of adsorption. The fitted results in Table 4 indicate that the pseudo-second-order kinetics model was chosen as the most favorable model for exhibiting the adsorption processes of glabridin on XT8-8, due to the good correlation obtained. For the adsorption of aromatic compounds on a synthetic polymeric adsorbent from aqueous solution, three necessary steps are often considered. And they include the external diffusion of the aromatic compound from water to the adsorbent surface, intraparticle diffusion in the pore of the adsorbent and adsorption on the adsorbent [41]. Additionally, the intra-particle diffusion is frequently the rate-limiting step. In the present study, the kinetic data are analyzed with an intra-particle diffusion model [42] proposed by Weber and Morris. As can be seen from Fig. 8, the fitting results of the Weber–Morris model at three different temperatures show similar characters having two linear segments followed by a plateau. In addition, none of the curves passed through the origin, which meant that the adsorption rate of glabridin on XT8-8 was controlled by both intraparticle diffusion and external diffusion. The kid,1 (see Table 5) evaluated are 2.53, 2.23 and 2.10 (mg/g min1/2 ) at 288, 303 and 318 K, respectively, proving that the intra-particle diffusion rate at a lower temperature is higher, and which is in accordance with the conclusion that the adsorption is an exothermic process.

Fig. 8. The fitted kinetic curves for the adsorption of XT8-8 towards glabridin according to the intra-particle diffusion model.

3.6. Adsorption mechanisms The specific surface area of the adsorbents plays an important role in the adsorption. Generally speaking, the larger the specific surface area of the adsorbent is, the greater the adsorption capacity is [43]. The specific surface area of XT2-10 and XT8-8 resin is measured to be 730.8 [31] and 905.2 m2 /g, respectively. The difference in the specific surface areas of the two resins may result in a difference in the adsorption capacity. In addition, the larger adsorption capacity of glabridin onto XT8-8 resin must have some other reasons except for the difference in the specific surface area. Polarity matching between the functional groups of the adsorbent and the polar groups of the adsorbate is also an important adsorption mechanism. It is known that XT2-10 resin has PS matrix, which is non-polar, while XT8-8 resin has acylamino group, which is polar. In addition, glabridin has polar hydroxy and epoxy groups on the benzene ring. Hence, XT2-10 resin exhibits lower adsorption capacity for glabridin than XT8-8 resin. On the whole, the remarkable adsorption behaviors of XT8-8 were due to its suitable structural design and modification. 3.7. Reusability It was found that the spent XT8-8 resin could be well desorbed by 100 mL 95% ethanol solution. The XT8-8 resin is then used repeatedly for five cycles in the continuous adsorption–desorption process, the equilibrium capacity is measured at every adsorption–desorption cycle. Fig. 9 indicates that the glabridin uptakes on the XT8-8 resin decreased to approximately 94.1% after five cycles of adsorption–desorption process, indicating that XT8-8 resin can be repeatedly used for glabridin adsorption without a significant reduction in adsorption performance. Thus, it can be concluded that the XT8-8 resin exhibited a remarkable reusability and the reliable regeneration performance of XT8-8 proved its practical applicability. 4. Conclusions Five acetylaniline-modified hypercrosslinked PS resins XT8-0, XT8-4, XT8-6, XT8-8 and XT8-10 were prepared by controlling the quantity of acetylaniline in the Friedel–Crafts reaction and they were shown to possess different pore textural property and surface functionality, indicative of their different adsorption performance towards glabridin. XT8-8 had the highest glabridin uptakes among

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Fig. 9. Effect of the regeneration cycles on the adsorption capacity of glabridin on XT8-8 at 288 K.

the five resins and it was superior to its precursor chloromethylated PS beads and the optimal commercial resin BMKB-1. The Freundlich model was more appropriate for fitting the equilibrium data than the Langmuir model, and the pseudo-second-order rate equation was more suitable for the kinetic data than the pseudo-first-order rate equation. The XT8-8 resin could be regenerated by 100 mL 95% ethanol solution and only 5.9% loss in the adsorption capacities of glabridin was observed after five cycles of adsorption–desorption process. In conclusion, suitable structural design and modification of XT8-8 played a significant role on its remarkable adsorption behaviors. Acknowledgments This research project was financially supported by the National Natural Sciences Foundation of China (NSFC No. 20974116, 21374128 and 21544013), the “Hundred Talents Program” of the Chinese Academy of Sciences (CAS), and the National High Technology Research and Development Program of China (863 No. 2014AA022203). The authors declare no competing financial interest(s). References [1] C. Simmler, G.F. Pauli, S.N. Chen, Phytochemistry and biological properties of glabridin, Fitoterapia 90 (2013) 160–184. [2] J. Vaya, P.A. Belinky, M. Aviram, Antioxidant constituents from licorice roots: isolation, structure elucidation and antioxidative capacity toward LDL oxidation, Free Radical Biol. Med. 23 (1997) 302–313. [3] T. Fukai, A. Marumo, K. Kaitou, T. Kanda, S. Terada, T. Nomura, Anti-Helicobacter pylori flavonoids from licorice extract, Life Sci. 71 (2002) 1449–1463. [4] S. Tamir, M. Eizenberg, D. Somjen, N. Stern, R. Shelach, A. Kaye, J. Vaya, Estrogenic and antiproliferative properties of glabridin from licorice in human breast cancer cells, Cancer Res. 60 (2000) 5704–5709. [5] H. Li, Y. Jia, M.C. Du, J.B. Fei, J. Zhao, Y. Cui, J.B. Li, Self-organization of honeycomb-like porous TiO2 films by means of the breath-figure method for surface modification of titanium implants, Chem. Eur. J. 19 (2013) 5306–5313. [6] Y. Cui, C. Tao, S.P. Zheng, Q. He, S.F. Ai, J.B. Li, Synthesis of thermosensitive PNIPAM-co-MBAA nanotubes by atom transfer radical polymerization within a porous membrane, Macromol. Rapid Comm. 26 (2005) 1552–1556. [7] Q.L. Zou, L. Zhang, X.H. Yan, A.H. Wang, G.H. Ma, J.B. Li, M. Helmuth, M. Stephen, Multifunctional porous microspheres based on peptide–porphyrin hierarchical co-assembly, Angew. Chem. Int. Ed. 53 (2014) 2366–2370. [8] Y. Su, X.H. Yan, A.H. Wang, J.B. Fei, Y. Cui, Q. He, J.B. Li, A peony-flower-like hierarchical mesocrystal formed by diphenylalanine, J. Mater. Chem. 20 (2010) 6734–6740. [9] Y. Li, J.H. Huang, J.B. Liu, S.G. Deng, X.Y. Lu, Adsorption of berberine hydrochloride, ligustrazine hydrochloride, colchicine, and matrine alkaloids on macroporous resins, J. Chem. Eng. Data 58 (2013) 1271–1279.

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