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Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution
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Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution Hongqi Zhang a, Tiantian Zhang a, Lei Tian b, Jiayan Qu a, Pange Liu a, Xiaomei Wang a,∗, Xu Zhang a,∗ a
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China State Key Laboratory of Separation Membranes and Membrane Processes/National International Joint Research Center for Separation Membranes, Tianjin Polytechnic University, Tianjin 300387, PR China
b
a r t i c l e
i n f o
Article history: Received 27 October 2017 Revised 3 March 2018 Accepted 2 April 2018 Available online xxx Keywords: Hierarchically macro–mesoporous structure Cross-linked polystyrene Friedel–Crafts Adsorption Salicylic acid
a b s t r a c t A novel amino modified hierarchically macro–mesoporous cross-linked polystyrene adsorbent (HP CLPSEDA) was fabricated via the combination of colloidal crystal templating method with Friedel–Crafts (F-C) technique, and adsorption performance of salicylic acid (SA) from aqueous solutions on HP CLPS-EDA was investigated. Compared with the amino modified three dimensional ordered macroporous CLPS (3DOM CLPS-EDA), the HP CLPS-EDA showed the largest adsorption capacity towards SA and maximum utilization of amino groups. Furthermore, the adsorption characteristics of HP CLPS-EDA including adsorption isotherms, adsorption kinetic, effect of solution pH on adsorption and the regenerations performance were researched and discussed. It was shown that, the maximum adsorption capacity of HP CLPS-EDA predicted by the Langmuir model was much higher than the most adsorbents reported in literatures. Additionally, the adsorption capacity of HP CLPS-EDA towards SA was well retained in a wide range of solution pH. After fifteen times regenerations, over 95% of the initial adsorption capacity of HP CLPS-EDA was preserved and the morphology was well retained. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Water pollution by phenolic compounds has received widespread concerns in recent years. Many methods and technical process have been developed to reduce the pollution, such as membrane separation [1], photo catalytic degradation [2], extraction [3], and adsorption [4–6]. Particularly, adsorption is considered to be one of the most efficient methods in wastewater treatment due to the high concentrating ability and reusability of the adsorbents. The design and synthesis of polymeric adsorbents have attracted wide attentions due to its favorable physicochemical stability, large adsorption capacity and facile functionalization [7– 9]. Recently, synthesis and applications of hierarchically structured porous materials have become a rapidly evolving field of current interest [10]. Compared with single-sized porous materials, hierarchically porous (HP) materials exhibit enhanced properties in mass transport and diffusion, which mainly result from the high surface ∗
Corresponding authors. E-mail addresses: [email protected] (X. Wang), [email protected] (X. Zhang).
area, large accessible space, and interconnected hierarchical porosity at different length scales. Mesopore or micropore is more likely for the location of active sites [11,12], and sorbents with abundant mesopores have demonstrated higher mass loading capacities irrespective of the surface area due to the improved accessibility of the adsorbate to the internal surfaces [13–15]. Additionally, macropores facilitate mass diffusion toward and away from these active sites and reduce transport limitations [10,11]. For example, Xiao et al. synthesized adsorption resins with macro-micro pore structures by Friedel–Crafts reaction base on the macroporous chloromethylated polystyrene resin [6]. Furthermore, high interconnectivity between different length scale pores is of great significance for mass transport through porous framework [16]. Consequently, to fabricate a novel hierarchically porous polymeric adsorbent with highly interconnected macro–mesoporous structure as well as high surface area seems to be an effective way of maximizing the utilization of active sites imbedded into porous frameworks (or adsorption efficiency) and improving adsorption capacity. Fabrication of three dimensionally ordered macroporous (3DOM) structure, meanwhile constructing mesopores on the ultrathin pore walls of 3DOM materials offer an efficient way for the construction of hierarchically macro–mesoporous architectures
https://doi.org/10.1016/j.jtice.2018.04.001 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001
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Fig. 1. Schematic synthesis of the HP CLPS-EDA.
with high surface area and interconnectivity [10–12]. It is worth mentioning that, to the best of our knowledge, there are few studies focused on incorporation of mesopores into the ultrathin pore walls of 3DOM resins to obtain polymeric adsorbents with large surface area and high interconnectivity, which maybe induce high adsorption capacity and improved adsorption efficiency. It may be challenging but meaningful to develop an effective functional hierarchically macro-mesoporous polymeric absorbent with high interconnectivity and large surface area. Herein, we propose a novel fabrication strategy of hierarchically macro–mesoporous cross-linked polystyrene (HP CLPS) through the combination of colloidal crystal templating (CCT) method with Friedel–Crafts (F-C) reaction technique. Cyanuric chloride was employed as the post cross-linking agent for the formation of mesopores [17]. SA was chosen as the model adsorbate due to its high toxicity and accumulation in the environment. Then HP CLPS resins were further functionalized with ethylenediamine (EDA) through a facial reaction, since polar amino groups can interact with SA by polarity matching [5,18]. Thus, a novel amino-modified hierarchically macro–mesoporous cross-linked polystyrene adsorbents (HP CLPS-EDA) was prepared accordingly. The ultrathin pore walls with adsorption sites (amino groups) and hierarchically macro– mesoporous structure will extremely improve the adsorption efficiency of the adsorption sites, accordingly increase the adsorption capacity. The overall fabrication procedure is shown in Fig. 1. The adsorption behaviors including the equilibrium, kinetics, effect of solution pH and reusability were also investigated in detail.
(99.9%) and salicylic acid (99.5%, SA) were obtained from Aladdin Industrial Corporation and used as received. 2.2. Preparation of 3DOM CLPS The preparation of 3DOM CLPS with a pore size of 245 nm by CCT method was performed according to our previous work [19]. The difference is that the mass percentage of DVB is 6% (w/w). 2.3. Fabrication of HP CLPS
2. Materials and methods
A representative example of fabricating HP CLPS is described as follows: anhydrous ferric chloride was used as F-C catalyst, cyanuric chloride as cross-linker, and nitrobenzene as solvent. The accurately weighted 0.4 g of 3DOM CLPS was swollen in 20 mL of nitrobenzene overnight at room temperature. Cyanuric chloride (0.7492 g) dissolved by 10 mL of nitrobenzene was then added into above mixture. Then, 2.0 g of anhydrous ferric chloride dissolved in another 10 mL nitrobenzene and was added into the reaction mixture. The reaction mixture was stirred for half an hour at 25 °C. After a short period of heating process, the temperature of the reaction mixture was increased to 100 °C and was stirred with magnetic stirring for 10 h. Subsequently, the resulting solid particles were filtrated from the solution, and then washed with ethanol and 1% of hydrochloric acid and anhydrous ethanol mixture until the effluents were transparent. Finally, the products were extracted by Soxhlet extractor with ethanol for 12 h and freeze-dried with 1,4-dioxane. The products named HP CLPS was obtained accordingly.
2.1. Chemicals and reagents
2.4. Functionalization of HP CLPS
Nitrobenzene (Tianjin Dengfeng Chemical Company, 99%) was ˚ before use. Styrene (St, 98%), dehydrated by molecular sieves (4 A) divinylbenzene (DVB, 80% isomer), and 2,2-azobisisobutyronitrile (AIBN) were purchased from Tianjin Guangfujingxi Chemical Company. St and DVB were dried over calcium hydride overnight and distilled under reduced pressure, then stored under argon at −10 °C. AIBN were purified by recrystallization in methanol. Cyanuric chloride (99%), ethylenediamine (EDA, 99%), ferric chloride
The chloromethylation reaction of HP CLPS was performed according to the method in Ref. [19], and the products were denoted as HP CLPS-CH2 Cl. A representative amino modification of the HP CLPS is described as follows: the accurately weighted HP CLPSCH2 Cl (0.5 g) were placed into a 100 mL of two-necked flask with a magnetic bar and vacuumed under ambient temperature for 2 h, then methanol (25 mL) was injected into the flask under vacuum conditions and stirred slowly. Afterwards, the reaction device was
Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001
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placed in an ice-water bath, and 25 mL of EDA were injected into the flask and stirred slowly for about 0.5 h. The reaction was carried out at 35 °C for 48 h. Afterwards, the resulting solid particles were filtrated from the solution, followed by wash with methanol for 3 times, and then extracted by Soxhlet extractor with methanol for 12 h. Finally, the products were freeze-dried with 1,4-dioxane. The amino-modified HP CLPS, named as HP CLPS-EDA. 2.5. Functionalization of 3DOM CLPS The chloromethylation and amino-modification reactions of 3DOM CLPS were the same as the preparation of HP CLPS-EDA described above. The products were denoted as 3DOM CLPS-EDA. 2.6. Characterization Hydrodynamic diameter measurements were performed on a Malvern Zetasizer Nano-ZS90. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker VECTOR-22 spectrometer at room temperature using KBr pellets. The samples were fractured and sputtered with gold in vacuum for scanning electron microscopy (SEM, FEI Nova NanoSEM450). The pore structures of the samples were measured by N2 adsorption-desorption isotherms at 77 K by ASAP 2020M + C surface area and porosity analyzer. The BET surface area was calculated according to the Brunauer– Emmett–Teller (BET) model. Moreover, the loading amount of amino groups were determined by analyzing nitrogen element with a Flash EA 1112 elemental analyzer. The concentrations of SA in aqueous solution were determined by using a UV–Vis spectroscopy (Cary 300, Varian) at a wavelength of 296 nm. A linear relationship between the absorbency and the SA concentration was obtained in the range of 4–100 mg/L. A well-fitted regression equation, A = 0.0245C + 0.02908, was obtained with the correlation coefficient R2 of 0.9997. 2.7. Adsorption isotherms
adjusted by hydrochloric acid or sodium hydroxide. A series of adsorption experiments towards SA were performed under the same conditions. 2.10. Regeneration of the HP adsorbent The reusability of the adsorbent was determined through consecutive adsorption–desorption for fifteen cycles using the same adsorbent under same conditions. After adsorption of SA every time, the SA loaded adsorbent was rinsed and immersed in 0.1 M sodium hydroxide for three times. Afterwards, the adsorbent was rinsed and washed with deionized water until neutral. 3. Results and discussion
The accurately weighted dry samples (about 0.1 g) were mixed with 100 mL of a series of SA aqueous solution (the concentrations of SA were ranging from 90 to 500 mg/L) into a 250 mL conical flask. After shaking of 24 h at different temperatures (293, 303, and 313 K), the suspension was filtered. The equilibrium concentration, Ce (mg/L), of the SA was measured. The equilibrium adsorption capacity, Qe (mg/g), was calculated with Eq (1)
Qe = (C0 − Ce )V/W
Fig. 2. FT-IR spectra of (a) 3DOM CLPS, (b) HP CLPS, (c) 3DOM CLPS-EDA, and (d) HP CLPS-EDA.
(1)
where C0 (mg/L) was the initial concentration of SA; Ce (mg/L) was the equilibrium concentration; V was the volume of the SA solution (L); W was the weight of the corresponding dry adsorbents (g). 2.8. Adsorption kinetic The accurately weighted dry samples (about 0.1 g) and 100 mL of SA solution with an initial concentration of 90 mg/L were introduced into a 250 mL cone-shaped flask and continuously shaken at 293, 303, and 313 K, respectively. The aqueous samples were taken from the shaker mixer at pre-set time intervals, and the concentration of SA was determined. 2.9. Effect of solution pH on adsorption The accurately weighed dry samples (about 0.1 g) were mixed with 100 mL of SA aqueous solution (about 500 mg/L) at different solutions pH in a 250 mL flask. The pH values of SA solutions were
3.1. Preparation and characterization of the HP adsorbent Fig. 2 shows the FT-IR spectra of 3DOM CLPS, HP CLPS, 3DOM CLPS-EDA, and HP CLPS-EDA, respectively. The FT-IR spectrum of the 3DOM CLPS (Fig. 2a) displays bands at 3085, 3060, and 3026 cm−1 demonstrating the C-H stretching of the benzene ring. Besides, the bands at 753 and 698 cm−1 obviously show the C-H vibrating signals of mono-substituted benzene, which indicate the 3DOM CLPS was successfully obtained. After the F-C reaction of CLPS with cyanuric chloride, characteristic peak of C=N bond vibration signals appear at 1680 cm−1 displayed in Fig. 2b, suggesting the successful introduction of cyanuric chloride to the polystyrene skeletons. Moreover, the new peaks at 1501 and 1630 cm−1 occurred in Figs. 2c and 2d are the characteristic of NH bond deformation and vibration, respectively, implying the successful amino functionalization of 3DOM CLPS and HP CLPS. The morphology of silica CCT, obtained by Stöber technique [20], is depicted in Fig. S1 in Supporting Information. It can be seen that uniform silica microspheres are close-packed into highly ordered CCTs by centrifugation. The average diameter of silica microspheres is 289 nm with a narrow particle distribution index (PDI) of 1.018 (Fig. S2 in Supporting Information). The morphology of 3DOM CLPS (Fig. 3a) shows uniformly arranged macropores with a diameter around 245 nm, which are smaller than that of the pristine silica template. The pore diameter contraction after removal of the rigid template is common in the preparation of 3DOM polymers and can be explained in terms of confinement effect of the polymer chains [21,22]. It is worth noting that the
Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001
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Fig. 3. SEM images of (a) 3DOM CLPS, (b) HP CLPS, (c) 3DOM CLPS-EDA, and (d) HP CLPS-EDA.
Fig. 4. (a) N2 adsorption–desorption and (b) pore size distribution curves for the 3DOM CLPS, HP CLPS and HP CLPS-EDA.
macropore of the HP CLPS (Fig. 3b) is much smoother than that of 3DOM CLPS with low cross-linking degree (insets of Fig. 3a and 3b), which also indicates the molecule chains of CLPS have been successfully further cross-linked by cyanuric chloride [23]. Additionally, after chemical modification with EDA (Fig. 3c and 3d), the 3D-ordered close-packed macropores are well preserved and few changes of aperture size exists. Furthermore, every macropore has twelve “windows” with neighboring cages, facing in multiple directions. The interconnected macropores and ultrathin pore walls ensure convenient diffusion channels for the substance transfer. The pore structures of the 3DOM CLPS, HP CLPS and HP CLPS-EDA were measured by N2 adsorption-desorption isotherms (Fig. 4). The BET surface area of 3DOM CLPS is only 49.07 m2 /g, which is much lower than that of HP CLPS (323.2 m2 /g). After introduction of polar amino groups into HP CLPS, the BET surface area decreased to 129.4 m2 /g, which agrees with the literatures reported [5,18]. Theoretically, the generation of meso-scale pores in the ultrathin pore walls improve the interconnectivity of the whole porous structures, leading to shorten diffusion paths and accelerated mass exchanges rate [24]. Consequently, the HP CLPS with high in-
terconnectivity, increased BET surface area may offer a superior platform for the synthesis of efficient adsorbents with decreased mass transfer resistance and increased utilization of active sites. Both the HP CLPS and 3DOM CLPS were chemically modified with EDA through a facial amination reaction, since the introduction of amino groups into porous framework may improve the polarity of the adsorbents (the adsorption capacities of 3DOM CLPS and HP CLPS are 8.6 and 66.2 mg/g, respectively). As shown in Table 1, the HP adsorbent exhibited much improved adsorption capacity and utilization of effective adsorptive than that of 3DOM CLPS-EDA, though possessing less loading amount of amino groups. These results confirm that hierarchically macro–mesoporous structure indeed provides a favorable platform for efficient adsorption than 3DOM CLPS.
3.2. Adsorption isotherms Fig. 5 displays the equilibrium isotherms of SA on HP CLPSEDA from aqueous solution under the temperature at 293, 303 and 313 K, respectively. The uptake of SA improves with increasing of
Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001
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Table 1 Adsorption data of the 3DOM CLPS-EDA and HP CLPS-EDA adsorbents towards SAa . Samples
Adsorption capacity (mmol/g)
Loading amount of amino groups (mmol/g)
Adsorption efficiencyb (mmol/mmol)
3DOM CLPS-EDA HP CLPS-EDA
2.33 2.97
6.73 4.59
0.35 0.65
a b
The adsorption conditions: C0 = 500 mg/g and T = 303 k. Adsorption efficiency = adsorption capacity towards SA/loading amount of amino groups.
Fig. 5. Equilibrium adsorption isotherms at 293, 303, and 313 K to SA onto the HP CLPS-EDA (fitted to Langmuir and Freundlich models are shown together with the experimental data points).
the equilibrium concentration as well as the increment of the temperature, implying that the adsorption is an endothermic process. Two commonly used models Langmuir and Frendlich models were adopted for fitting the obtained experimental results (Fig. 5). The data were performed in the nonlinear fittings by these two models as shown in Supporting Information. The typical parameters and the correlation coefficients R2 are listed in Table S1, in Supporting Information. Both of the Langmuir and Freundlich models are proven to be suitable for fitting the equilibrium data since R2 > 0.98 and Langmuir model appears to be more fitted for the equilibrium data due to the higher R2 . The Qm was predicted to be 485.8, 509.8 and 528.6 mg/g at 293, 303 and 313 K, respectively. The values of the maximum adsorption capacity are much higher than the reported activated carbon (351.0 mg/g) [25], amine modified nano-dendritic SiO2 -Al2 O3 particles (256.1 mg/g) [26], some commercial polymeric adsorbents such as X-5, XAD-7 and AB-8 [27], hyper-cross-linked resins [5,9], as well as some novel polymeric adsorbents developed in recent years (396.8 and 151.7 mg/g, respectively) [6,28]. The outstandingly large adsorption capacity of HP CLPS-EDA towards SA can be attributed to the hierarchically macro–mesoporous structure in addition to the large number of the amino groups anchored in porous skeletons. Besides, the KL improves with increasing of temperature, indicating that the adsorption affinity of SA to the active sites of HP CLP-EDA were enhanced with improvement of the temperature. 3.3. Adsorption kinetics Fig. 6 presents the adsorption kinetic curve of HP CLPS-EDA and 3DOM CLPS-EDA towards SA with the initial concentration of 90 mg/L at 303 K. Although the amount of amino groups of HP CLPS-EDA is much less than that of 3DOM CLPS-EDA (Table 1), the adsorption equilibrium time for HP CLPS-EDA is much shorter than that of 3DOM CLPS-EDA (about 40 and 110 min, respectively), man-
Fig. 6. Kinetic adsorption curve of HP CLPS-EDA adsorbents towards SA at 293, 303, and 313 K, together with that of 3DOM CLPS-EDA at 303 K. The initial concentration is 90 mg/L.
ifesting a relatively faster adsorption rate for SA on HP CLPS-EDA. It can be ascribed to the large BET surface area of HP CLPS-EDA together with the improved interconnectivity of the hierarchically macro–mesoporous structure, which induce faster diffusion rate in the pore of HP CLPS-EDA adsorbent. The results further confirm that HP CLPS provide a superior platform for efficient mass delivery and high utilization of effective groups loaded into porous frameworks. The adsorption kinetic of HP CLPS-EDA at 293 and 313 K were also studied and the results are shown together in Fig. 6. It can be seen that the adsorption capacity improves rapidly with increasing of time and reach equilibrium within almost 40 min. The fast adsorption rate of HP CLPS-EDA is also attributed to the hierarchically macro–mesoporous structure with high interconnectivity. Furthermore, a higher temperature induces a shorter equilibrium time, which further prove a high temperature is favorable for the adsorption process. The Lagergren pseudo-first-order and pseudo second-order were adopted to fit adsorption kinetic data since they were widely used for the analysis of the adsorption kinetic data (Supporting Information). The fitness is estimated in terms of the coefficient of determination, R2 . The fitted kinetic parameters are listed in Table S2, in Supporting Information. The results indicate that SA adsorption kinetic on HP CLPS-EDA is better fitted by the pseudosecond order model since the higher R2 values, and the calculated Qe (cal.) values are closer to the experimental data than those of the pseudo-first order model. The values of k1 and k2 increase with the increasing of the temperature, which further prove a high temperature is favorable for the adsorption process. 3.4. Effect of solution pH on the adsorption The state of HP CLPS-EDA adsorbent (the adsorptive amino groups introduced to porous skeletons) and SA can be affected by the solution pH, both of which influence the adsorption capacity
Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001
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Fig. 7. Effect of the solution pH on the adsorption of HP CLPS-EDA towards SA in aqueous solution at 303 K, and the initial concentration of SA is 500 mg/L.
[5]. Thus, the investigation of the relationship between the adsorption capacity and the solution pH is essential. As is observed from Fig. 7, the adsorption capacity of HP CLPS-EDA is sensitive to the solution pH. There is first a tendency of increase then a decrease with the solution pH increase. When the pH is around 3, the adsorption capacity towards SA reaches maximum. Additionally, the adsorption capacity of HP CLPS-EDA can be retained at a relative high value in a wide range until the solution pH is over 9. It can be explained that when the solution pH is lower than 2.98 (the pKa1 and pKa2 of SA are 2.98 and 13.1, respectively), the amino groups are protonated in acidic solution, while the ionization degree of SA is retarded and more SA will exist in solution as molecules. Consequently, with the increasing of pH value in the early stage, the adsorption capacity was gradually enhanced. As the solution pH further increases, the protonation of the amino groups is reduced even though the ionization of SA enhances. Therefore, the adsorption capacity towards SA tends to be constant and fluctuates in the solution pH ranging from 4 to 9. When the pH is more than 11, the adsorption capacity sharply decreased, because there are few protonated amino groups for adsorbing ionized SA. The results indicate that the charge interaction is the dominated driving force for the adsorption of SA onto HP CLPS-EDA in aqueous solution. Furthermore, the HP CLPS-EDA adsorbent shows promising prospects in a wide range of pH solutions in practical applications.
Fig. 8. (a) Adsorption-desorption cycle of HP CLPS-EDA towards SA from aqueous solution and (b) the SEM image of HP CLPS-EDA after fifteen adsorption–desorption cycles.
amount of amino groups. The adsorption isotherms revealed that Langmuir model is better fitted the isotherms. The kinetics studies indicated that the adsorption process of HP CLPS-EDA to SA followed the pseudo-second order model. The effect of solution pH on adsorption suggested that the adsorption capacity of HP CLPSEDA can be retained at a relative high level in a wide range of solution pH. After fifteen times adsorption-desorption cycles, the regenerated HP CLPS-EDA exhibited 95% of the initial adsorption capacity. All the results suggest that the HP CLPS offers a superior platform with reduced mass transfer resistance and improved utilization of active sites. The HP CLPS-EDA adsorbent shows great potential as a new generation of efficient adsorbent. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 51573038, 51403049, 50903027) and the Natural Science Foundation of Hebei Province (Nos. E2016202261 and E2017202036). Supplementary materials
3.5. The regeneration of HP CLPS-EDA The reusability of the adsorbents is of great significance in practical applications. Thereby, the reusability of HP CLPS-EDA was evaluated by means of continuous repeated adsorption-desorption experiments. The results of the adsorption performance for every cycle are displayed in Fig. 8a. It can be seen that over 95% of the original adsorption capacity of HP CLPS-EDA are maintained after every adsorption–desorption cycle. Additionally, the morphology of the adsorbent after 15 times regeneration cycles was perfectly preserved (Fig. 8b). These results prove that the HP CLPS-EDA framework to be excellent stable and can be served as a new generation of efficient adsorbent. 4. Conclusions A novel HP CLPS-EDA adsorbent with hierarchical macro– mesoporous structure was fabricated through the F-C reaction technique and amination reaction. The HP CLPS-EDA showed the optimal adsorption capacity and utilization of active groups due to its appropriate pore structure of HP CLPS backbone as well as the
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[19] Huang Y, Zhang H, Sun M, Wang X, Li G, Liu P, Zhang X. Quaternized three-dimensionally ordered macroporous cross-linked polystyrene and its adsorption character toward salicylic acid in aqueous solution. Sep Sci Technol 2014;49:2586–94. [20] Stöber W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in the micron size range. J Colloid Interface Sci 1968;26:62–9. [21] Russell TP. Chance encounters. Science 2001;293:446–7. [22] Rong J, Yang Z. Preparation and characterization of confined atactic polystyrene in ordered silica/polystyrene composites. Macromol Mater Eng 2002;287:11–15. [23] Huang J, Wang X, Patil PD, Tang J, Chen L, Liu Y. Synthesis, characterization and adsorption properties of an amide-modified hyper-cross-linked resin. RSC Adv 2014;4:41172–8. [24] Phillips KR, England GT, Sunny S, Shirman E, Shirman T, Vogel N, Aizenberg J. A colloidoscope of colloid-based porous materials and their uses. Chem Soc Rev 2016;45:281–322. [25] Otero M, Grande CA, Rodrigues AE. Adsorption of salicylic acid onto polymeric adsorbents and activated charcoal. React Funct Polym 2004;60:203–13. [26] Arshadi M, Mousavinia F, Abdolmaleki MK, Amiri MJ, Khalafi-Nezhad A. Removal of salicylic acid as an emerging contaminant by a polar nano-dendritic adsorbent from aqueous media. J Colloid Interface Sci 2017;493:138–49. [27] Huang J, Zha H, Jin X, Deng S. Efficient adsorptive removal of phenol by a diethylenetriamine-modified hypercrosslinked styrene–divinylbenzene (PS) resin from aqueous solution. Chem Eng J 2012;195:40–8. [28] Fu Z, Huang J. Polar hyper-cross-linked resin with abundant micropores/mesopores and its enhanced adsorption toward salicylic acid: equilibrium, kinetics, and dynamic operation. Fluid Phase Equilib 2017;438:1–9.
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Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution Hongqi Zhang a, Tiantian Zhang a, Lei Tian b, Jiayan Qu a, Pange Liu a, Xiaomei Wang a,∗, Xu Zhang a,∗ a
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China State Key Laboratory of Separation Membranes and Membrane Processes/National International Joint Research Center for Separation Membranes, Tianjin Polytechnic University, Tianjin 300387, PR China
b
a r t i c l e
i n f o
Article history: Received 27 October 2017 Revised 3 March 2018 Accepted 2 April 2018 Available online xxx Keywords: Hierarchically macro–mesoporous structure Cross-linked polystyrene Friedel–Crafts Adsorption Salicylic acid
a b s t r a c t A novel amino modified hierarchically macro–mesoporous cross-linked polystyrene adsorbent (HP CLPSEDA) was fabricated via the combination of colloidal crystal templating method with Friedel–Crafts (F-C) technique, and adsorption performance of salicylic acid (SA) from aqueous solutions on HP CLPS-EDA was investigated. Compared with the amino modified three dimensional ordered macroporous CLPS (3DOM CLPS-EDA), the HP CLPS-EDA showed the largest adsorption capacity towards SA and maximum utilization of amino groups. Furthermore, the adsorption characteristics of HP CLPS-EDA including adsorption isotherms, adsorption kinetic, effect of solution pH on adsorption and the regenerations performance were researched and discussed. It was shown that, the maximum adsorption capacity of HP CLPS-EDA predicted by the Langmuir model was much higher than the most adsorbents reported in literatures. Additionally, the adsorption capacity of HP CLPS-EDA towards SA was well retained in a wide range of solution pH. After fifteen times regenerations, over 95% of the initial adsorption capacity of HP CLPS-EDA was preserved and the morphology was well retained. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Water pollution by phenolic compounds has received widespread concerns in recent years. Many methods and technical process have been developed to reduce the pollution, such as membrane separation [1], photo catalytic degradation [2], extraction [3], and adsorption [4–6]. Particularly, adsorption is considered to be one of the most efficient methods in wastewater treatment due to the high concentrating ability and reusability of the adsorbents. The design and synthesis of polymeric adsorbents have attracted wide attentions due to its favorable physicochemical stability, large adsorption capacity and facile functionalization [7– 9]. Recently, synthesis and applications of hierarchically structured porous materials have become a rapidly evolving field of current interest [10]. Compared with single-sized porous materials, hierarchically porous (HP) materials exhibit enhanced properties in mass transport and diffusion, which mainly result from the high surface ∗
Corresponding authors. E-mail addresses: [email protected] (X. Wang), [email protected] (X. Zhang).
area, large accessible space, and interconnected hierarchical porosity at different length scales. Mesopore or micropore is more likely for the location of active sites [11,12], and sorbents with abundant mesopores have demonstrated higher mass loading capacities irrespective of the surface area due to the improved accessibility of the adsorbate to the internal surfaces [13–15]. Additionally, macropores facilitate mass diffusion toward and away from these active sites and reduce transport limitations [10,11]. For example, Xiao et al. synthesized adsorption resins with macro-micro pore structures by Friedel–Crafts reaction base on the macroporous chloromethylated polystyrene resin [6]. Furthermore, high interconnectivity between different length scale pores is of great significance for mass transport through porous framework [16]. Consequently, to fabricate a novel hierarchically porous polymeric adsorbent with highly interconnected macro–mesoporous structure as well as high surface area seems to be an effective way of maximizing the utilization of active sites imbedded into porous frameworks (or adsorption efficiency) and improving adsorption capacity. Fabrication of three dimensionally ordered macroporous (3DOM) structure, meanwhile constructing mesopores on the ultrathin pore walls of 3DOM materials offer an efficient way for the construction of hierarchically macro–mesoporous architectures
https://doi.org/10.1016/j.jtice.2018.04.001 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001
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Fig. 1. Schematic synthesis of the HP CLPS-EDA.
with high surface area and interconnectivity [10–12]. It is worth mentioning that, to the best of our knowledge, there are few studies focused on incorporation of mesopores into the ultrathin pore walls of 3DOM resins to obtain polymeric adsorbents with large surface area and high interconnectivity, which maybe induce high adsorption capacity and improved adsorption efficiency. It may be challenging but meaningful to develop an effective functional hierarchically macro-mesoporous polymeric absorbent with high interconnectivity and large surface area. Herein, we propose a novel fabrication strategy of hierarchically macro–mesoporous cross-linked polystyrene (HP CLPS) through the combination of colloidal crystal templating (CCT) method with Friedel–Crafts (F-C) reaction technique. Cyanuric chloride was employed as the post cross-linking agent for the formation of mesopores [17]. SA was chosen as the model adsorbate due to its high toxicity and accumulation in the environment. Then HP CLPS resins were further functionalized with ethylenediamine (EDA) through a facial reaction, since polar amino groups can interact with SA by polarity matching [5,18]. Thus, a novel amino-modified hierarchically macro–mesoporous cross-linked polystyrene adsorbents (HP CLPS-EDA) was prepared accordingly. The ultrathin pore walls with adsorption sites (amino groups) and hierarchically macro– mesoporous structure will extremely improve the adsorption efficiency of the adsorption sites, accordingly increase the adsorption capacity. The overall fabrication procedure is shown in Fig. 1. The adsorption behaviors including the equilibrium, kinetics, effect of solution pH and reusability were also investigated in detail.
(99.9%) and salicylic acid (99.5%, SA) were obtained from Aladdin Industrial Corporation and used as received. 2.2. Preparation of 3DOM CLPS The preparation of 3DOM CLPS with a pore size of 245 nm by CCT method was performed according to our previous work [19]. The difference is that the mass percentage of DVB is 6% (w/w). 2.3. Fabrication of HP CLPS
2. Materials and methods
A representative example of fabricating HP CLPS is described as follows: anhydrous ferric chloride was used as F-C catalyst, cyanuric chloride as cross-linker, and nitrobenzene as solvent. The accurately weighted 0.4 g of 3DOM CLPS was swollen in 20 mL of nitrobenzene overnight at room temperature. Cyanuric chloride (0.7492 g) dissolved by 10 mL of nitrobenzene was then added into above mixture. Then, 2.0 g of anhydrous ferric chloride dissolved in another 10 mL nitrobenzene and was added into the reaction mixture. The reaction mixture was stirred for half an hour at 25 °C. After a short period of heating process, the temperature of the reaction mixture was increased to 100 °C and was stirred with magnetic stirring for 10 h. Subsequently, the resulting solid particles were filtrated from the solution, and then washed with ethanol and 1% of hydrochloric acid and anhydrous ethanol mixture until the effluents were transparent. Finally, the products were extracted by Soxhlet extractor with ethanol for 12 h and freeze-dried with 1,4-dioxane. The products named HP CLPS was obtained accordingly.
2.1. Chemicals and reagents
2.4. Functionalization of HP CLPS
Nitrobenzene (Tianjin Dengfeng Chemical Company, 99%) was ˚ before use. Styrene (St, 98%), dehydrated by molecular sieves (4 A) divinylbenzene (DVB, 80% isomer), and 2,2-azobisisobutyronitrile (AIBN) were purchased from Tianjin Guangfujingxi Chemical Company. St and DVB were dried over calcium hydride overnight and distilled under reduced pressure, then stored under argon at −10 °C. AIBN were purified by recrystallization in methanol. Cyanuric chloride (99%), ethylenediamine (EDA, 99%), ferric chloride
The chloromethylation reaction of HP CLPS was performed according to the method in Ref. [19], and the products were denoted as HP CLPS-CH2 Cl. A representative amino modification of the HP CLPS is described as follows: the accurately weighted HP CLPSCH2 Cl (0.5 g) were placed into a 100 mL of two-necked flask with a magnetic bar and vacuumed under ambient temperature for 2 h, then methanol (25 mL) was injected into the flask under vacuum conditions and stirred slowly. Afterwards, the reaction device was
Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001
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placed in an ice-water bath, and 25 mL of EDA were injected into the flask and stirred slowly for about 0.5 h. The reaction was carried out at 35 °C for 48 h. Afterwards, the resulting solid particles were filtrated from the solution, followed by wash with methanol for 3 times, and then extracted by Soxhlet extractor with methanol for 12 h. Finally, the products were freeze-dried with 1,4-dioxane. The amino-modified HP CLPS, named as HP CLPS-EDA. 2.5. Functionalization of 3DOM CLPS The chloromethylation and amino-modification reactions of 3DOM CLPS were the same as the preparation of HP CLPS-EDA described above. The products were denoted as 3DOM CLPS-EDA. 2.6. Characterization Hydrodynamic diameter measurements were performed on a Malvern Zetasizer Nano-ZS90. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker VECTOR-22 spectrometer at room temperature using KBr pellets. The samples were fractured and sputtered with gold in vacuum for scanning electron microscopy (SEM, FEI Nova NanoSEM450). The pore structures of the samples were measured by N2 adsorption-desorption isotherms at 77 K by ASAP 2020M + C surface area and porosity analyzer. The BET surface area was calculated according to the Brunauer– Emmett–Teller (BET) model. Moreover, the loading amount of amino groups were determined by analyzing nitrogen element with a Flash EA 1112 elemental analyzer. The concentrations of SA in aqueous solution were determined by using a UV–Vis spectroscopy (Cary 300, Varian) at a wavelength of 296 nm. A linear relationship between the absorbency and the SA concentration was obtained in the range of 4–100 mg/L. A well-fitted regression equation, A = 0.0245C + 0.02908, was obtained with the correlation coefficient R2 of 0.9997. 2.7. Adsorption isotherms
adjusted by hydrochloric acid or sodium hydroxide. A series of adsorption experiments towards SA were performed under the same conditions. 2.10. Regeneration of the HP adsorbent The reusability of the adsorbent was determined through consecutive adsorption–desorption for fifteen cycles using the same adsorbent under same conditions. After adsorption of SA every time, the SA loaded adsorbent was rinsed and immersed in 0.1 M sodium hydroxide for three times. Afterwards, the adsorbent was rinsed and washed with deionized water until neutral. 3. Results and discussion
The accurately weighted dry samples (about 0.1 g) were mixed with 100 mL of a series of SA aqueous solution (the concentrations of SA were ranging from 90 to 500 mg/L) into a 250 mL conical flask. After shaking of 24 h at different temperatures (293, 303, and 313 K), the suspension was filtered. The equilibrium concentration, Ce (mg/L), of the SA was measured. The equilibrium adsorption capacity, Qe (mg/g), was calculated with Eq (1)
Qe = (C0 − Ce )V/W
Fig. 2. FT-IR spectra of (a) 3DOM CLPS, (b) HP CLPS, (c) 3DOM CLPS-EDA, and (d) HP CLPS-EDA.
(1)
where C0 (mg/L) was the initial concentration of SA; Ce (mg/L) was the equilibrium concentration; V was the volume of the SA solution (L); W was the weight of the corresponding dry adsorbents (g). 2.8. Adsorption kinetic The accurately weighted dry samples (about 0.1 g) and 100 mL of SA solution with an initial concentration of 90 mg/L were introduced into a 250 mL cone-shaped flask and continuously shaken at 293, 303, and 313 K, respectively. The aqueous samples were taken from the shaker mixer at pre-set time intervals, and the concentration of SA was determined. 2.9. Effect of solution pH on adsorption The accurately weighed dry samples (about 0.1 g) were mixed with 100 mL of SA aqueous solution (about 500 mg/L) at different solutions pH in a 250 mL flask. The pH values of SA solutions were
3.1. Preparation and characterization of the HP adsorbent Fig. 2 shows the FT-IR spectra of 3DOM CLPS, HP CLPS, 3DOM CLPS-EDA, and HP CLPS-EDA, respectively. The FT-IR spectrum of the 3DOM CLPS (Fig. 2a) displays bands at 3085, 3060, and 3026 cm−1 demonstrating the C-H stretching of the benzene ring. Besides, the bands at 753 and 698 cm−1 obviously show the C-H vibrating signals of mono-substituted benzene, which indicate the 3DOM CLPS was successfully obtained. After the F-C reaction of CLPS with cyanuric chloride, characteristic peak of C=N bond vibration signals appear at 1680 cm−1 displayed in Fig. 2b, suggesting the successful introduction of cyanuric chloride to the polystyrene skeletons. Moreover, the new peaks at 1501 and 1630 cm−1 occurred in Figs. 2c and 2d are the characteristic of NH bond deformation and vibration, respectively, implying the successful amino functionalization of 3DOM CLPS and HP CLPS. The morphology of silica CCT, obtained by Stöber technique [20], is depicted in Fig. S1 in Supporting Information. It can be seen that uniform silica microspheres are close-packed into highly ordered CCTs by centrifugation. The average diameter of silica microspheres is 289 nm with a narrow particle distribution index (PDI) of 1.018 (Fig. S2 in Supporting Information). The morphology of 3DOM CLPS (Fig. 3a) shows uniformly arranged macropores with a diameter around 245 nm, which are smaller than that of the pristine silica template. The pore diameter contraction after removal of the rigid template is common in the preparation of 3DOM polymers and can be explained in terms of confinement effect of the polymer chains [21,22]. It is worth noting that the
Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001
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Fig. 3. SEM images of (a) 3DOM CLPS, (b) HP CLPS, (c) 3DOM CLPS-EDA, and (d) HP CLPS-EDA.
Fig. 4. (a) N2 adsorption–desorption and (b) pore size distribution curves for the 3DOM CLPS, HP CLPS and HP CLPS-EDA.
macropore of the HP CLPS (Fig. 3b) is much smoother than that of 3DOM CLPS with low cross-linking degree (insets of Fig. 3a and 3b), which also indicates the molecule chains of CLPS have been successfully further cross-linked by cyanuric chloride [23]. Additionally, after chemical modification with EDA (Fig. 3c and 3d), the 3D-ordered close-packed macropores are well preserved and few changes of aperture size exists. Furthermore, every macropore has twelve “windows” with neighboring cages, facing in multiple directions. The interconnected macropores and ultrathin pore walls ensure convenient diffusion channels for the substance transfer. The pore structures of the 3DOM CLPS, HP CLPS and HP CLPS-EDA were measured by N2 adsorption-desorption isotherms (Fig. 4). The BET surface area of 3DOM CLPS is only 49.07 m2 /g, which is much lower than that of HP CLPS (323.2 m2 /g). After introduction of polar amino groups into HP CLPS, the BET surface area decreased to 129.4 m2 /g, which agrees with the literatures reported [5,18]. Theoretically, the generation of meso-scale pores in the ultrathin pore walls improve the interconnectivity of the whole porous structures, leading to shorten diffusion paths and accelerated mass exchanges rate [24]. Consequently, the HP CLPS with high in-
terconnectivity, increased BET surface area may offer a superior platform for the synthesis of efficient adsorbents with decreased mass transfer resistance and increased utilization of active sites. Both the HP CLPS and 3DOM CLPS were chemically modified with EDA through a facial amination reaction, since the introduction of amino groups into porous framework may improve the polarity of the adsorbents (the adsorption capacities of 3DOM CLPS and HP CLPS are 8.6 and 66.2 mg/g, respectively). As shown in Table 1, the HP adsorbent exhibited much improved adsorption capacity and utilization of effective adsorptive than that of 3DOM CLPS-EDA, though possessing less loading amount of amino groups. These results confirm that hierarchically macro–mesoporous structure indeed provides a favorable platform for efficient adsorption than 3DOM CLPS.
3.2. Adsorption isotherms Fig. 5 displays the equilibrium isotherms of SA on HP CLPSEDA from aqueous solution under the temperature at 293, 303 and 313 K, respectively. The uptake of SA improves with increasing of
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Table 1 Adsorption data of the 3DOM CLPS-EDA and HP CLPS-EDA adsorbents towards SAa . Samples
Adsorption capacity (mmol/g)
Loading amount of amino groups (mmol/g)
Adsorption efficiencyb (mmol/mmol)
3DOM CLPS-EDA HP CLPS-EDA
2.33 2.97
6.73 4.59
0.35 0.65
a b
The adsorption conditions: C0 = 500 mg/g and T = 303 k. Adsorption efficiency = adsorption capacity towards SA/loading amount of amino groups.
Fig. 5. Equilibrium adsorption isotherms at 293, 303, and 313 K to SA onto the HP CLPS-EDA (fitted to Langmuir and Freundlich models are shown together with the experimental data points).
the equilibrium concentration as well as the increment of the temperature, implying that the adsorption is an endothermic process. Two commonly used models Langmuir and Frendlich models were adopted for fitting the obtained experimental results (Fig. 5). The data were performed in the nonlinear fittings by these two models as shown in Supporting Information. The typical parameters and the correlation coefficients R2 are listed in Table S1, in Supporting Information. Both of the Langmuir and Freundlich models are proven to be suitable for fitting the equilibrium data since R2 > 0.98 and Langmuir model appears to be more fitted for the equilibrium data due to the higher R2 . The Qm was predicted to be 485.8, 509.8 and 528.6 mg/g at 293, 303 and 313 K, respectively. The values of the maximum adsorption capacity are much higher than the reported activated carbon (351.0 mg/g) [25], amine modified nano-dendritic SiO2 -Al2 O3 particles (256.1 mg/g) [26], some commercial polymeric adsorbents such as X-5, XAD-7 and AB-8 [27], hyper-cross-linked resins [5,9], as well as some novel polymeric adsorbents developed in recent years (396.8 and 151.7 mg/g, respectively) [6,28]. The outstandingly large adsorption capacity of HP CLPS-EDA towards SA can be attributed to the hierarchically macro–mesoporous structure in addition to the large number of the amino groups anchored in porous skeletons. Besides, the KL improves with increasing of temperature, indicating that the adsorption affinity of SA to the active sites of HP CLP-EDA were enhanced with improvement of the temperature. 3.3. Adsorption kinetics Fig. 6 presents the adsorption kinetic curve of HP CLPS-EDA and 3DOM CLPS-EDA towards SA with the initial concentration of 90 mg/L at 303 K. Although the amount of amino groups of HP CLPS-EDA is much less than that of 3DOM CLPS-EDA (Table 1), the adsorption equilibrium time for HP CLPS-EDA is much shorter than that of 3DOM CLPS-EDA (about 40 and 110 min, respectively), man-
Fig. 6. Kinetic adsorption curve of HP CLPS-EDA adsorbents towards SA at 293, 303, and 313 K, together with that of 3DOM CLPS-EDA at 303 K. The initial concentration is 90 mg/L.
ifesting a relatively faster adsorption rate for SA on HP CLPS-EDA. It can be ascribed to the large BET surface area of HP CLPS-EDA together with the improved interconnectivity of the hierarchically macro–mesoporous structure, which induce faster diffusion rate in the pore of HP CLPS-EDA adsorbent. The results further confirm that HP CLPS provide a superior platform for efficient mass delivery and high utilization of effective groups loaded into porous frameworks. The adsorption kinetic of HP CLPS-EDA at 293 and 313 K were also studied and the results are shown together in Fig. 6. It can be seen that the adsorption capacity improves rapidly with increasing of time and reach equilibrium within almost 40 min. The fast adsorption rate of HP CLPS-EDA is also attributed to the hierarchically macro–mesoporous structure with high interconnectivity. Furthermore, a higher temperature induces a shorter equilibrium time, which further prove a high temperature is favorable for the adsorption process. The Lagergren pseudo-first-order and pseudo second-order were adopted to fit adsorption kinetic data since they were widely used for the analysis of the adsorption kinetic data (Supporting Information). The fitness is estimated in terms of the coefficient of determination, R2 . The fitted kinetic parameters are listed in Table S2, in Supporting Information. The results indicate that SA adsorption kinetic on HP CLPS-EDA is better fitted by the pseudosecond order model since the higher R2 values, and the calculated Qe (cal.) values are closer to the experimental data than those of the pseudo-first order model. The values of k1 and k2 increase with the increasing of the temperature, which further prove a high temperature is favorable for the adsorption process. 3.4. Effect of solution pH on the adsorption The state of HP CLPS-EDA adsorbent (the adsorptive amino groups introduced to porous skeletons) and SA can be affected by the solution pH, both of which influence the adsorption capacity
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Fig. 7. Effect of the solution pH on the adsorption of HP CLPS-EDA towards SA in aqueous solution at 303 K, and the initial concentration of SA is 500 mg/L.
[5]. Thus, the investigation of the relationship between the adsorption capacity and the solution pH is essential. As is observed from Fig. 7, the adsorption capacity of HP CLPS-EDA is sensitive to the solution pH. There is first a tendency of increase then a decrease with the solution pH increase. When the pH is around 3, the adsorption capacity towards SA reaches maximum. Additionally, the adsorption capacity of HP CLPS-EDA can be retained at a relative high value in a wide range until the solution pH is over 9. It can be explained that when the solution pH is lower than 2.98 (the pKa1 and pKa2 of SA are 2.98 and 13.1, respectively), the amino groups are protonated in acidic solution, while the ionization degree of SA is retarded and more SA will exist in solution as molecules. Consequently, with the increasing of pH value in the early stage, the adsorption capacity was gradually enhanced. As the solution pH further increases, the protonation of the amino groups is reduced even though the ionization of SA enhances. Therefore, the adsorption capacity towards SA tends to be constant and fluctuates in the solution pH ranging from 4 to 9. When the pH is more than 11, the adsorption capacity sharply decreased, because there are few protonated amino groups for adsorbing ionized SA. The results indicate that the charge interaction is the dominated driving force for the adsorption of SA onto HP CLPS-EDA in aqueous solution. Furthermore, the HP CLPS-EDA adsorbent shows promising prospects in a wide range of pH solutions in practical applications.
Fig. 8. (a) Adsorption-desorption cycle of HP CLPS-EDA towards SA from aqueous solution and (b) the SEM image of HP CLPS-EDA after fifteen adsorption–desorption cycles.
amount of amino groups. The adsorption isotherms revealed that Langmuir model is better fitted the isotherms. The kinetics studies indicated that the adsorption process of HP CLPS-EDA to SA followed the pseudo-second order model. The effect of solution pH on adsorption suggested that the adsorption capacity of HP CLPSEDA can be retained at a relative high level in a wide range of solution pH. After fifteen times adsorption-desorption cycles, the regenerated HP CLPS-EDA exhibited 95% of the initial adsorption capacity. All the results suggest that the HP CLPS offers a superior platform with reduced mass transfer resistance and improved utilization of active sites. The HP CLPS-EDA adsorbent shows great potential as a new generation of efficient adsorbent. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 51573038, 51403049, 50903027) and the Natural Science Foundation of Hebei Province (Nos. E2016202261 and E2017202036). Supplementary materials
3.5. The regeneration of HP CLPS-EDA The reusability of the adsorbents is of great significance in practical applications. Thereby, the reusability of HP CLPS-EDA was evaluated by means of continuous repeated adsorption-desorption experiments. The results of the adsorption performance for every cycle are displayed in Fig. 8a. It can be seen that over 95% of the original adsorption capacity of HP CLPS-EDA are maintained after every adsorption–desorption cycle. Additionally, the morphology of the adsorbent after 15 times regeneration cycles was perfectly preserved (Fig. 8b). These results prove that the HP CLPS-EDA framework to be excellent stable and can be served as a new generation of efficient adsorbent. 4. Conclusions A novel HP CLPS-EDA adsorbent with hierarchical macro– mesoporous structure was fabricated through the F-C reaction technique and amination reaction. The HP CLPS-EDA showed the optimal adsorption capacity and utilization of active groups due to its appropriate pore structure of HP CLPS backbone as well as the
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Please cite this article as: H. Zhang et al., Amino-modified hierarchically macro-mesoporous cross-linked polystyrene: A novel adsorbent for removal of salicylic acid from aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.04.001