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Preparation and characterization of polar polymeric adsorbents with high surface area for the removal of phenol from water
Journal of Hazardous Materials 177 (2010) 773–780
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Preparation and characterization of polar polymeric adsorbents with high surface area for the removal of phenol from water Xiaowei Zeng, Tingjun Yu, Peng Wang, Ronghua Yuan, Qing Wen, Yunge Fan ∗ , Chunhong Wang, Rongfu Shi Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 14 October 2009 Received in revised form 22 December 2009 Accepted 23 December 2009 Available online 4 January 2010 Keywords: Polar adsorbent High surface area Pendant vinyl groups Post-crosslinking Adsorption Phenol
a b s t r a c t Preparation of methyl methacrylate (MMA)/divinylbenzene (DVB) and ethylene glycol dimethacrylate (EGDMA)/DVB copolymers via suspension polymerization yielded precursors which possess residual vinyl groups. Post-crosslinking of appropriate dichloroethane swollen precursors without external crosslinking agent in the presence of anhydrous ferric chloride (FeCl3 ) yielded post-crosslinked resins with high surface area and suitable polarity. FT-IR spectrum indicated that increasing the proportion of MMA or EGDMA in monomer mixtures notably reduces the amount of the pendant vinyl groups onto the matrix of the precursors. Furthermore, the pendant vinyl groups of precursors were almost absent when the content of MMA and EGDMA increased to 40 mol% and 20 mol% in the monomers, respectively. The specific surface areas and pore volumes of copolymers showed a remarkable increase after postcrosslinking. Experimental results showed that isotherms of phenol adsorption onto these polymeric adsorbents could be represented by Freundlich model and Langmuir model reasonably. PDE-5pc exhibited higher adsorption capacity of phenol than other adsorbents, which resulted from synergistic effect of larger specific surface area and polar groups onto the network. Column adsorption/desorption dynamic curves suggested that PDE-5pc is a potential candidate for treatment of chemical effluent containing phenol and phenolic pollutants. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Contamination of surface water and groundwater with hazardous compounds has attracted increasing attention in recent decades all over the world. Phenol and substituted phenols are widely found in the effluents from pesticides, synthetic rubber, plastic, pharmaceuticals, petrochemicals, and other industries [1]. Due to its acute toxicity and potential accumulation in the environment, phenol and its derivatives have already been listed as one of the high priority concerns. Therefore, various methods have been proposed for the efficient removal of phenols from wastewater. Many methods and technical processes including catalytic oxidation, biodegradation, solvent extraction and adsorption have been developed to treat the phenol-containing effluents [2–5]. On account of the high concentrating ability of typical adsorbents, adsorption is proven to be one of the most attractive and effective techniques for purification and separation in wastewater treatment [6]. Porous materials such as activated carbon and polymeric adsorbents are widely used in water and wastewater treatment for the removal of organic and inorganic contaminants because of its large
∗ Corresponding author. Tel.: +86 2223503935; fax: +86 2223503935. E-mail address: [email protected] (Y. Fan). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.12.100
specific surface area and predominant proportion of micropores [7]. In recent years, owing to its better mechanical strength and feasible regeneration under mild conditions, polymeric adsorbents have increasingly been used for efficient removal of specific aromatic compounds from contaminated water [8,9]. The most widely used commercial resin Amberlite XAD-4 was referred as one of the best polymeric adsorbents for the removal of phenol and phenol compounds from wastewater [10]. However, the extreme hydrophobicity of the surface and its hydrophobic interactions with the adsorbates lead to poor interaction with polar compounds and lower its adsorption capacity. These drawbacks could be largely overcome by introducing polar functional groups onto the adsorbent matrix [11]. Polar adsorbents could be synthesized by copolymerization from a balanced ratio of a hydrophilic monomer and a crosslinker. Some of these adsorbents are commercially available, for instance, Amberlite XAD-7 and XAD-8 (Room & Haas), which are based on polymethacrylate; Oasis HLB (Waters) is a macroporous poly (N-vinylpyrrolidone–divinylbenzene) (PVP–DVB) copolymer. Moreover, Abselut Nexus (Varian) is another hydrophilic commercial sorbent which based on the copolymerization of methacrylate–divinylbenzene (MA–DVB) [12]. Recently, Trochimczuk’s group synthesized several polar polymeric sorbents by copolymerization of cyanomethyl styrene
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(CMSt)/divinylbenzene (DVB) [13], acrylonitrile (AN)/DVB and methacrylonitrile (MAN)/DVB [14,15]. These adsorbents were tested in the sorption of phenols and it was found that the best sorption properties were for the adsorbents with a proportion of 50:50. Therefore, the researchers demonstrated that both polarity and specific surface area were equally important. Marce and co-authors synthesized a series of hydrophilic sorbents by copolymerization of a polar monomer and a crosslinking agent (DVB). These sorbents were based on 4-vinylpyridine–divinylbenzene (4VP–DVB) [16], 4-vinylimidazole–divinylbenzene (4VIm–DVB) and N-vinylimidazole–divinylbenzene (NVIm–DVB) [17,18]. The specific surface area of these resins ranges from several to 700 m2 /g. When these resins were tested as sorbents for solidphase extraction for extracting polar compounds, the best results were obtained for NVIm–DVB, which has both the highest specific area and the quite high nitrogen content. Thus, both the proportion of polar functional groups and specific surface area are important for the retention of polar compounds [12]. Another approach of introducing polarity into a sorbent is the chemically modification of hydrophobic polymer skeleton with a suitable polar moiety. Fritz et al. first introduced this method by modifying resins with sulfonic, acetyl and hydroxymethyl groups [19,20]. More recently, Salih et al. modified a hydrophobic poly (styrene–divinylbenzene) with amine, nitrile and carboxyl groups [21], but the specific surface area of these polar polymeric sorbents are not very high. Therefore, in view of the above, an ideal material for adsorption of phenolic compounds would be a resin which has both polarity and large special surface area. Polymerization techniques to prepare macroporous polymer particles with large specific surface areas have come through three generations. The first-generation technique was prepared by divinylbenzene (DVB) copolymerization in the presence of the inert solvent. The specific surface area of the resulting product was often limited by the purity of DVB and the mechanism of the pore formation induced by phase separation in the process of polymerization [22]. The second-generation technique, called hypercrosslinked resin, was first introduced in the 1970s by Davankov et al. [23,24] and developed further by Jerabek and co-authors [25,26]. The hypercrosslinked resins were obtained by an extensive post-crosslinking of a preformed linear polystyrene or low-crosslinked polystyrene using a bifunctional crosslinking agent and a Friedel-Crafts catalyst. In this way, methylene bridges which reinforced the structure of the polymer matrix were formed between polymer chains. Alternatively, it is possible to use divinylbenzene–vinylbenzyl chloride (DVB–VBC) copolymer as the precursor in the presence of Friedel-Crafts catalyst and solvent [27,28]. This kind of hypercrosslinked resin exhibits an extremely high specific surface area and excellent sorption properties. However, the preparation of hypercrosslinked adsorbents is faced with a number of problems; the most serious one is the utilization of recognized carcinogen, chloromethyl methyl ether, as additional crosslinker in post-crosslinking reaction [29], and the production cost of the monomer vinylbenzyl chloride is rather high. The third-generation technique was the objective of the present research of this paper, just by post-crosslinking of the pendant vinyl groups on the starting macroporous styrene–divinylbenzene copolymers. The method of synthesis the post-crosslinked polymeric adsorbent by residual double bonds was first proposed by Ando et al. [30]. The specific surface area of the macroporous polymeric materials was great increased by Friedel-Crafts catalyzed reaction of the residual double bonds with neighboring benzene rings without an additional crosslinking agent. Recently, Yan’s group [31] and Jerabek’s group [29] investigated the post-crosslinking of the starting macroporous styrene–divinylbenzene copolymers via pendant vinyl groups using anhydrous ferric chloride as catalyst. Hao et al.
have synthesized the uniform micrometer-sized polydivinylbenzene microspheres with high surface areas via the Friedel-Crafts catalyst of anhydrous aluminum chloride [32]. However, the precursors of synthesizing such post-crosslinked adsorbents were non-polar, and their extreme hydrophobic surface was disadvantageous in industrial applications, such as extraction of polar compounds. The post-crosslinking of polar sorbents has been rarely recorded in the literature, despite the fact that such materials may be of more generic value in industry. Recently, we developed the technique of post-crosslinking reaction of pendant vinyl groups to prepare a novel polar polymeric adsorbent [33]. The resultant polymer particles have excellent adsorption capacity of phenol aqueous solution because the adsorbent possesses high special surface area and polarity. However, the relationship of polarity and special surface area has not been investigated in our previous work. In the present work, in order to study the effect of post-crosslinking reaction of pendant vinyl groups on the precursor polymer particles, we prepared a series of MMA-co-DVB and EGDMA-co-DVB adsorbents by using different contents of MMA and EGDMA. In the case of the N2 adsorption experiments the surface area, pore volume and average pore diameter were calculated from the adsorption branch of the isotherm. Phenol was selected as a model phenolic compound for further adsorption study. 2. Experimental 2.1. Materials Divinylbenzene (DVB 80% grade), methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) were kindly supplied by Nankai Hecheng S&T (Tianjin, China). All of them were washed with 10% aqueous sodium hydroxide and water and then dried over anhydrous magnesium sulfate prior to use. The initiators 2,2 azobisisobutyronitrile (AIBN) was supplied by Concord Technology (Tianjin, China) and was recrystallized from methanol. Poly(vinyl alcohol) (PVA) (88% hydrolyzed, Mn ∼125 000), anhydrous ferric chloride (FeCl3 ), 1,2-dichloroethane (DCE), toluene, heptane, acetone, methanol, phenol, sodium chloride were purchased from Tianjin Chemical Co. (Tianjin, China) and were used without further purification. The water used in the present work was deionized water without pH adjustment. 2.2. Preparation of the precursor copolymers The precursor copolymer beads were obtained by the usual suspension polymerization method in a 1000 mL three-necked round-bottomed flask reactor fitted with a mechanical stirrer, a reflux condenser and a thermometer [16]. At room temperature, the flask was charged with an aqueous phase containing 1% PVA and 10% NaCl. The organic phase, a mixture of MMA or EGDMA, DVB, the porogen, and the initiator (1.0 mol% in relation to the monomer mixture), was added to the aqueous phase. Suitable stirring rate was adjusted, and the polymerization process was performed at 353 K for 12 h. The composition of monomer mixtures is shown in Table 1. The prepared copolymer beads were collected, washed and extracted with acetone in Soxhlet apparatus for 10 h and then dried under vacuum at 333 K for 8 h. The sieving copolymer beads with fraction in the range 75–150 m were selected for further studies. 2.3. Post-crosslinking of precursor beads Scheme 1 presented the elaboration process of preparation the post-crosslinked adsorbent. The post-crosslinking reaction was carried out in 1,2-dichloroethane, using FeCl3 as Friedel-Crafts catalyst in amount corresponding to 20% of the starting copolymer
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Table 1 Polymerization conditions and typical properties of MMA-co-DVB and EGDMA-co-DVB copolymersa . Adsorbent
Compositions (mol/mol)
BET surface area (m2 /g)b
Pore volume (mL/g)c
Average pore diameter (nm)d
MMA/DVBe PDVB PDVBpc PDM-10 PDM-10pc PDM-20 PDM-20pc PDM-40 PDM-40pc
0/100 0/100 10/90 10/90 20/80 20/80 40/60 40/60
859.2 1128.6 762.6 988.4 638.6 821.8 582.8 601.5
1.61 1.72 1.42 1.59 1.08 1.32 1.06 1.10
10.03 9.33 10.85 9.07 9.40 8.63 9.87 9.82
EGDMA/DVB PDE-5 PDE-5pc PDE-10 PDE-10pc PDE-20 PDE-20pc
5/95 5/95 10/90 10/90 20/80 20/80
810.8 1018.2 714.7 811.5 698.8 716.2
1.51 1.66 1.34 1.46 1.49 1.53
10.62 9.76 11.73 10.47 11.29 11.16
Water uptake (g/g)
Swelling ratio
Water
Toluene
Methanol
1.58 2.03 1.64 1.98 1.71 1.85 1.92 1.89
1.19 1.06 1.25 1.10 1.29 1.11 1.36 1.32
1.42 1.23 1.51 1.26 1.56 1.31 1.70 1.65
1.26 1.15 1.29 1.16 1.32 1.12 1.41 1.38
1.72 2.16 1.77 1.93 2.02 1.97
1.32 1.12 1.27 1.13 1.35 1.31
1.46 1.19 1.50 1.24 1.49 1.42
1.23 1.06 1.31 1.11 1.28 1.22
a The organic phase/aqueous phase ratio was 1/4; the organic phase consisted of a monomer mixture (DVB and MMA/EGDMA), a porogen mixture (toluene and heptane), at a 1:2 ratio (v/v); the porogen toluene and heptane at a 2:1 ratio (v/v). b BET surface area for repeat polymer batches was reproducible within (5%). c BJH cumulative pore volume for pores between 1.7 and 300 nm in diameter. d BJH average pore diameter. e 80% grade monomer.
weight [29]. The reaction temperature was 353 K and the reaction time was 8 h. The copolymer beads after reaction were washed with acetone, acetone/0.1 M HCl (1:1 v/v), and water and then dried under vacuum at 333 K for 8 h. 2.4. Adsorption experiments of adsorbents The static adsorption of phenol was performed at the temperature of 298 K. Batch adsorption experiments were performed as follows: 0.100 g of dry adsorbent was introduced into a 100 mL conical flask, it was firstly wetted with 1.0 mL of methanol and then rinsed three times with deionized water. 25 mL of phenol aqueous solution containing a known concentration was added into each flask. The flasks were sealed and then transferred to an incubator shaker with thermostat (DSHZ-300, Taicang Laboratorial Equipment Factory, Jiangsu, China) and shaken under 120 rpm for 12 h at desired temperature to ensure that the adsorption process reached equilibrium. The equilibrium adsorption capacity Qe (mg/g) was calculated with Eq. (1): Qe =
V (C0 − Ce ) W
(1)
where V is the volume of solution (L), W is the mass of dry adsorbent (g), C0 and Ce (mg/L) denote the initial and equilibrium concentration of phenol in aqueous solution, respectively. Dynamic adsorption was carried out with a glass column (8 mm diameter and 200 mm length) packed with 5.0 mL (wet volume) post-crosslinked resin PDE-5pc at ambient temperature. Phenol aqueous solution of
3000 mg/L was passed through the column at a flow rate of 4.0 BV/h (BV is bed volume) until the adsorption curve completed, then industrial alcohol was used for desorbing resin PDE-5pc adsorbed phenol at a flow of 2.0 BV/h followed by 1.0 BV of deionized water. Breakthrough adsorption capacity and the total capacity were calculated based on the total amount of phenol removed when the concentration of the effluent from the column reached 5% and nearly 100% of the initial concentration, respectively. 2.5. Characterization The concentrations of phenol solutions were determined by using a UV-spectrophotometer (Purkinje General, Beijing, China) at the wavelength of 270 nm. The swelling ratio in water (toluene, methanol) defined as the ratio of the volume occupied by the resin when swollen in water (toluene, methanol) to its volume in the dry state at room temperature, was determined using a small measuring cylinder. Water uptake of the adsorbents was measured by the centrifugation method [13]. The specific surface area and the pore size distribution of the adsorbents were calculated, respectively by BET and BJH methods via the nitrogen adsorption and desorption curves at 77 K using a Micromeritics Tristar 3000 automatic surface area and porosity analyzer (Micromeritics Instrument Corp., USA). Before the BET surface area measurement, the adsorbents were outgassed at 333 K for 24 h on the degas port of the analyzer. Infrared spectra of the polymeric adsorbent before and after the post-crosslinking reaction were obtained from a MAGNA-560 FT-IR spectrometer (Nicolet, USA) with a pellet of powdered potas-
Scheme 1. Conceptual illustration of preparation of the post-crosslinked adsorbents.
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sium bromide and adsorbent in the range of 500–4000 cm−1 . The surface morphology of the copolymer particles was investigated by scanning electron microscopy (SEM) using a Scanning Electron Microscope (Hitachi 3500N). 3. Results and discussion 3.1. FT-IR spectra of precursor and post-crosslinked particles The FT-IR spectra of precursors and post-crosslinked polymeric resins MMA-co-DVB are shown in Fig. 1. Clearly, except for polymers PDVB and PDVBpc, all the polymers have a strong band with frequency at 1732 cm−1 corresponding to the strong stretching vibration of ester groups. From Fig. 1, it can be seen that the two characteristic bands for residual vinyl groups (CH CH2 ) at 1630 and 990 cm−1 almost disappeared after post-crosslinking reaction, and the peak at 902 cm−1 is also weakened. The phenomenon was in good agreement with other reports [29,34,35], which demonstrated that residual vinyl groups in starting polymeric resins were consumed during the reaction with the help of anhydrous ferric chloride. Fig. 1 illustrates that the content of MMA exerts strong influence on the amount of pendant vinyl groups onto starting copolymers. In present work, in order to probe that issue, we obtained four types of starting copolymer beads with a progressively increasing MMA concentration in monomer mixtures. As a result, increasing the proportion of MMA in monomer mixtures notably reduces the amount of the pendant vinyl groups onto the matrix of the starting copolymers. Furthermore, the residual vinyl groups of starting copolymers were almost absent when the content of MMA up to 40 mol% in the monomer mixtures, which may be attributed to the crosslinking density of monomer mixtures and the reactivity coefficients of monomers MMA and DVB. Sherrington co-authors [36] have reported that the level of residual unreacted vinyl groups of each resin could be determined by the degree of crosslinking, which was measured via using solid state 13 C nuclear magnetic resonance (NMR) techniques. As in the previous case, decreasing the amount of the crosslinking monomer (DVB) reduces that of the residual vinyl groups. The reactivity coefficients (rMMA and rDVB ) reflect the inherent tendencies of a radical to react with its own monomer relative to the comonomer. The reactivity coefficients of MMA (rMMA ) and DVB (rDVB ) are less than 1 at various temperatures [37], which indicate the monomer is more likely to copolymerize with comonomer. He and co-authors [37] and Cheng et al. [38] have extensively studied that the reaction kinetics of pendant vinyl groups in the divinylbenzene suspension polymer-
Fig. 1. FT-IR spectra of MMA-co-DVB adsorbents before and after post-crosslinking reaction.
Fig. 2. FT-IR spectra of EGDMA-co-DVB adsorbents before and after postcrosslinking reaction.
ization. They believed that the activity of the second double bonds of the crosslinker DVB was rather low. Therefore, a lot of pendant vinyl groups remained on the polymer after the polymerization was completed. After a polar monomer MMA was added into the DVB polymerization system, more second double bonds would be consumed in the process of copolymerization reaction. Thereby, the residual vinyl groups of precursor resins would diminish correspondingly. For the purpose of improving the mechanical rigidity and special surface area of resins, a series of polar crosslinking monomers such as EGDMA were used to replace MMA in the polymerization mixture. Despite the copolymerization of monomer mixtures EGDMA and DVB was reported in a lot of literature [39,40], the study of pendant vinyl groups on the skeleton of resins did not seem to be recorded. Thereby an investigation was carried out in the way which was employed in the study of the MMA-co-DVB adsorbents. The results of FT-IR spectra of precursors and post-crosslinked polymeric resins EGDMA-co-DVB are summarized in Fig. 2. As can be seen from Figs. 1 and 2, the IR spectra of copolymers EGDMA-coDVB and MMA-co-DVB are almost identical, which indicate the precursors of EGDMA-co-DVB possess residual vinyl groups and they disappeared after post-crosslinking reaction as well. 3.2. Resins porosity characteristics Table 1 lists some important characteristics of MMA-co-DVB resins. The resins were prepared by using technical DVB and MMA. After post-crosslinking modification, both the specific surface area and pore volume of the precursor resins PDVB, PDM-10, PDM20 showed a remarkable increase. We can also discover that the increase of specific surface area is more significant for precursor resins with higher DVB content than with lower DVB content. That is because the residual vinyl group content of the precursor resins decreases with the decrease of the DVB content, which leads to less new crosslinking bonds created by the Friedel-Crafts reaction. However, those phenomena are not obvious in the precursor resin PDM-40, which can be attributed to the fact that there are almost no pendant vinyl groups onto PDM-40. Similar results were reported by Nyhus et al. [41] and Yan and co-authors [42]. Resins EGDMAco-DVB were prepared by the copolymerization of EGDMA and DVB, and some typical properties are presented in Table 1. Similarly, both the surface area and the pore volume of these starting polymeric resins increase after post-crosslinking reaction. Nevertheless, when the content of EGDMA is 20 mol%, there is almost no increase in special surface area and pore volume. Fig. 3(a) and (b) shows the specific surface area of copolymers before and after
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Fig. 3. Dependence of the specific surface area of precursors and post-crosslinked copolymers reaction on the ratio of polar monomer to divinylbenzene (a) MMA/DVB and (b) EGDMA/DVB. Specific surface area: (A) before post-crosslinking reaction Sb ; (B) after post-crosslinking reaction Sa ; (C) Sa −Sb .
the post-crosslinking reaction has correlation with the ratio of polar monomer MMA or EGDMA to DVB. Fig. 3 indicates that the addition of specific surface area declines with the decrease of DVB content. A much better representation of the pore structure in resins is of course given by the pore size distribution. The pore size distribution curves for the copolymers PDM-10 and PDE-10 prior to and after post-crosslinking reaction as revealed by the BJH (Barret, Joyner, and Halenda) method [43] are shown in Fig. 4(a) and (b). Compared with the precursors PDM-10 and PDE-10, post-crosslinked copolymers PDM-10pc and PDE-10pc presented more micropores after post-crosslinking reaction. The result that the average pore diameter of precursors in Table 1 decreased after post-crosslinking modification could be attributed to the increase of crosslinking degree. One of the most prominent properties of precursor and postcrosslinked resins is their response to solvents such as water, toluene and methanol, and which is demonstrated by the swelling ratio of solvents in Table 1. Moreover it shows the changing characteristic of precursor resins networks after post-crosslinking reaction. As is shown in Table 1, not only do the swelling ratio data for the thermodynamically compatible solvent, toluene, decrease relative to the precursor in the case of post-crosslinked resin, but also the two thermodynamically “poor” solvents, n-hexane and water, also exhibit a moderate decline. The phenomenon of different swelling ratio in various solvents demonstrates that the stability and rigidity of skeleton structure of the precursor resins increased after post-crosslinking reaction, which is significant for practical application. The water uptake of resins typically approaches 2 g/g. Before the water uptake of the resin was mea-
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Fig. 4. Pore size distribution curves of resins obtained by the nitrogen adsorption technique using BJH method; (a) precursor PDM-10 and its post-crosslinked derivative PDM-10pc (b) precursor PDE-10 and its post-crosslinked derivative PDE-10pc.
sured, it was progressively swollen by toluene, toluene/acetone, acetone, acetone/water and finally rinsed three times with deionized water [13]. (Resins do not swell directly in water, thus it is necessary to wash them in a solvent to increase some expansion of the polymeric network.) 3.3. Scanning electron microscopy (SEM) Scanning electron micrographs of two typical precursor resins (PDM-10 and PDE-10) and their post-crosslinking derivatives (PDM-10pc and PDE-10pc) are presented in Fig. 5. It is noteworthy that the precursors become more highly porous after the Friedel-Crafts modification. This surface morphology change could be fairly understandable if we consider that some of the newly formed crosslinking bonds may be “glued” together. The additional crosslinks increased the rigidity of the polymer beads, which indicated that the specific surface area of precursor resins enhanced significantly after post-crosslinking reaction. Similar results were reported elsewhere by Jerabek and co-authors [29] and Gong and co-authors [32]. 3.4. Adsorption isotherms Fig. 6(a)–(c) presented the equilibrium adsorption isotherms of phenol from aqueous solution onto copolymers MMA-co-DVB and EGDMA-co-DVB at the temperature of 298 K. Post-crosslinked adsorbents exhibited higher adsorption capacity than precursor adsorbents, which resulted from the specific surface area has great increase after post-crosslinking reaction.
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Fig. 5. Scanning electron micrographs of copolymer beads before and after Friedel-Crafts post-crosslinking: (A), PDM-10; (B), PDM-10pc; (C), PDE-10; (D), PDE-10pc. Magnification is 300× and 20k×.
The Freundlich equation and Langmuir equation were employed to fit the equilibrium adsorption data. The Freundlich model and Langmuir model can be expressed as Eq. (2) [44] and Eq. (3) [45–47], respectively. log Qe = log KF +
1 log Ce n
(2)
Ce 1 Ce = + KL Qm Qm Qe
(3)
where KF is a characteristic parameter as a measure of adsorption capacity ((mg/g)/(mg/L)1/n ), whereas n refers to as adsorption intensity and is an indicator of the favorableness of the adsorbent/adsorbate system. Qm is the maximum adsorption capacity (mg/g), and KL is a parameter which relates to the adsorption energy (L/g). The values of isotherm constants at 298 K are listed in Table 2. It is observed that the equilibrium data fitted both the Freundlich and Langmuir model well with correlation coefficient (R2 ) are larger than 0.99. The exponent (n) is larger than 1 in all cases, which demonstrates the adsorption of phenol onto adsorbents are favorable. Table 2 shows that PDE-5pc has better adsorption capacity than other adsorbents. The maximum adsorption capacity Qm of phenol onto PDT-5pc is 250.6 mg/g, which is larger than the results reported by other researchers and commercial polymeric adsorbents Amberlite XAD-4 and AB-8 [13,14,33]. It is commonly known that adsorption capacity of adsorbent from aqueous solution is dominated by many factors and the adsorbate–adsorbent interaction will play an important role. The Table 2 Freundlich and Langmuir isotherm parameters of phenol adsorption onto different adsorbents at 298 K. Adsorbent
PDVB PDVBpc PDM-10 PDM-10pc PDM-20 PDM-20pc PDM-40 PDM-40pc PDE-5 PDE-5pc PDE-10 PDE-10pc PDE-20 PDE-20pc
Freundlich model
Langmuir model 2
KF
n
R
KL
Qm
R2
3.394 6.937 3.811 7.507 3.506 5.490 3.802 4.422 4.047 8.326 3.608 5.477 3.986 4.815
2.069 2.245 2.116 2.478 2.104 2.468 2.192 2.385 2.132 2.438 2.103 2.451 2.211 2.408
0.995 0.999 0.995 0.998 0.996 0.998 0.999 0.997 0.998 0.999 0.998 0.999 0.997 0.998
1.424 1.515 1.429 1.716 1.432 2.115 1.268 1.730 1.301 1.587 1.395 1.967 1.306 1.608
190.1 231.5 194.2 228.1 185.2 197.6 183.8 184.9 202.3 250.6 193.0 212.4 185.5 187.3
0.999 0.996 0.994 0.996 0.992 0.995 0.998 0.997 0.998 0.999 0.995 0.998 0.996 0.999
effective interaction causing different adsorption behaviors of different adsorbates mainly includes: (1) hydrophobic interaction. The interactions with the adsorbates are basically by the – sites of the aromatic rings that make up the sorbent structure; (2) van der Waals force. It mainly includes dispersion interaction, dipole–dipole and dipole–induced dipole interactions; (3) hydrogen bonding interactions [12,13,48]. The adsorbing capacity of phenol for post-crosslinking derivatives is larger than precursors. This can be explained that the specific surface area of starting copolymer is greatly increased after post-crosslinking reaction. Accordingly, more – sites are available to interact with the adsorbate. Thus, the post-crosslinking derivatives possess of larger number of – sites which are helpful for adsorption of phenol than initial copolymers. Compared with the post-crosslinked resin PDE-5pc, the adsorbent PDVBpc has larger specific surface area, but its adsorption capacity for phenol on PDVBpc is lower than PDE-5pc, which further demonstrates that besides specific surface area, adsorption capacity would be affected by other factors. The surface of PDE-5pc resin contains polar groups of ester groups, and the polarity matching between the PDE-5pc resin and phenol may partially account for the increase in the adsorption capacities [13,18]. Besides the difference in polarity, the hydrogen bonding should also be taken into account, which occurs between the carbonyl group on PDE5pc and the hydroxyl group of phenol, and plays an important role in the adsorption of phenol [49]. For the precursors PDM-20, PDM-40, PDE-10, PDE-20, post-crosslinked adsorbents PDM-20pc, PDM-40pc, PDE-10pc, PDE-20pc, they have lower specific surface area then adsorbents PDVBpc, PDM-10pc, PDE-5pc, although the formation of hydrogen bonding could increase the adsorption capacity of PDM-20, PDM-40, PDE-10, PDE-20, PDM-20pc, PDM40pc, PDE-10pc, PDE-20pc, it cannot compensate for the decrease in adsorption capacity induced by the loss of the specific surface area. In a word, adsorption capacity could be affected by several factors, including specific surface area, adsorbent polarity and pore structure. 3.5. Dynamic adsorption and desorption Because of the satisfactory adsorption capacity and affinity for phenol onto the post-crosslinked copolymer PDE-5pc, it is hopeful that PDE-5pc will be developed as a polymeric adsorbent for the removal and recovery of phenolic contaminants from drinking water or industrial effluent. It is necessary to test the dynamic adsorption and desorption. The results of glass column dynamic adsorption of phenol on PDE-5pc are shown in Fig. 7, where CV is
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Fig. 7. Adsorption and desorption dynamic curves of phenol onto PDE-5pc at room temperature.
4. Conclusions A series of novel post-crosslinked polar polymeric adsorbents were prepared to enhance adsorption of phenol from aqueous solution. Both the specific surface area and the pore volume of polymeric adsorbents increased remarkably after post-crosslinking by Friedel-Crafts reaction of pendant vinyl groups onto precursor copolymers. The adsorbent PDE-5pc exhibited the highest adsorption capacity of phenol, which was traceable to its large specific surface area as well as the polar groups on its network. The adsorption isotherm data of all adsorbents were in good agreement with the Freundlich model and Langmuir model. On the basis of those results, further work on the syntheses of adsorbents with higher specific surface areas and higher polarity is planned. Acknowledgment The authors gratefully acknowledge the Tianjin Municipal Science and Technology Commission (Grant no. 09JCYBJC13600). References
Fig. 6. Comparison of adsorption isotherms of phenol (a) onto PDVB and PDVBpc at 298 K; (b) onto copolymers MMA-co-DVB at 298 K; (c) onto copolymers EGDMAco-DVB at 298 K.
the concentration at different bed volumes of the effluent (mg/L), CI is the concentration of initial solution (mg/L), and CR is the concentration at different bed volumes of regenerative reagent industrial alcohol (g/L). The results of breakthrough adsorption capacity and the total adsorption capacity are 56.8 and 60.1 mg/mL wet resin, respectively. As can be seen from Fig. 7, the shape of the adsorption curve is very sharp indicating that the adsorption of phenol onto PDE-5pc reached equilibrium quickly after leakage, which is crucial for industrial applications. Industrial alcohol was used to desorb phenol from PDE-5pc resin column. Nearly 100% regeneration efficiency was achieved by 3.0 BV industrial ethanol at room temperature (Fig. 7).
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Preparation and characterization of polar polymeric adsorbents with high surface area for the removal of phenol from water Xiaowei Zeng, Tingjun Yu, Peng Wang, Ronghua Yuan, Qing Wen, Yunge Fan ∗ , Chunhong Wang, Rongfu Shi Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 14 October 2009 Received in revised form 22 December 2009 Accepted 23 December 2009 Available online 4 January 2010 Keywords: Polar adsorbent High surface area Pendant vinyl groups Post-crosslinking Adsorption Phenol
a b s t r a c t Preparation of methyl methacrylate (MMA)/divinylbenzene (DVB) and ethylene glycol dimethacrylate (EGDMA)/DVB copolymers via suspension polymerization yielded precursors which possess residual vinyl groups. Post-crosslinking of appropriate dichloroethane swollen precursors without external crosslinking agent in the presence of anhydrous ferric chloride (FeCl3 ) yielded post-crosslinked resins with high surface area and suitable polarity. FT-IR spectrum indicated that increasing the proportion of MMA or EGDMA in monomer mixtures notably reduces the amount of the pendant vinyl groups onto the matrix of the precursors. Furthermore, the pendant vinyl groups of precursors were almost absent when the content of MMA and EGDMA increased to 40 mol% and 20 mol% in the monomers, respectively. The specific surface areas and pore volumes of copolymers showed a remarkable increase after postcrosslinking. Experimental results showed that isotherms of phenol adsorption onto these polymeric adsorbents could be represented by Freundlich model and Langmuir model reasonably. PDE-5pc exhibited higher adsorption capacity of phenol than other adsorbents, which resulted from synergistic effect of larger specific surface area and polar groups onto the network. Column adsorption/desorption dynamic curves suggested that PDE-5pc is a potential candidate for treatment of chemical effluent containing phenol and phenolic pollutants. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Contamination of surface water and groundwater with hazardous compounds has attracted increasing attention in recent decades all over the world. Phenol and substituted phenols are widely found in the effluents from pesticides, synthetic rubber, plastic, pharmaceuticals, petrochemicals, and other industries [1]. Due to its acute toxicity and potential accumulation in the environment, phenol and its derivatives have already been listed as one of the high priority concerns. Therefore, various methods have been proposed for the efficient removal of phenols from wastewater. Many methods and technical processes including catalytic oxidation, biodegradation, solvent extraction and adsorption have been developed to treat the phenol-containing effluents [2–5]. On account of the high concentrating ability of typical adsorbents, adsorption is proven to be one of the most attractive and effective techniques for purification and separation in wastewater treatment [6]. Porous materials such as activated carbon and polymeric adsorbents are widely used in water and wastewater treatment for the removal of organic and inorganic contaminants because of its large
∗ Corresponding author. Tel.: +86 2223503935; fax: +86 2223503935. E-mail address: [email protected] (Y. Fan). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.12.100
specific surface area and predominant proportion of micropores [7]. In recent years, owing to its better mechanical strength and feasible regeneration under mild conditions, polymeric adsorbents have increasingly been used for efficient removal of specific aromatic compounds from contaminated water [8,9]. The most widely used commercial resin Amberlite XAD-4 was referred as one of the best polymeric adsorbents for the removal of phenol and phenol compounds from wastewater [10]. However, the extreme hydrophobicity of the surface and its hydrophobic interactions with the adsorbates lead to poor interaction with polar compounds and lower its adsorption capacity. These drawbacks could be largely overcome by introducing polar functional groups onto the adsorbent matrix [11]. Polar adsorbents could be synthesized by copolymerization from a balanced ratio of a hydrophilic monomer and a crosslinker. Some of these adsorbents are commercially available, for instance, Amberlite XAD-7 and XAD-8 (Room & Haas), which are based on polymethacrylate; Oasis HLB (Waters) is a macroporous poly (N-vinylpyrrolidone–divinylbenzene) (PVP–DVB) copolymer. Moreover, Abselut Nexus (Varian) is another hydrophilic commercial sorbent which based on the copolymerization of methacrylate–divinylbenzene (MA–DVB) [12]. Recently, Trochimczuk’s group synthesized several polar polymeric sorbents by copolymerization of cyanomethyl styrene
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(CMSt)/divinylbenzene (DVB) [13], acrylonitrile (AN)/DVB and methacrylonitrile (MAN)/DVB [14,15]. These adsorbents were tested in the sorption of phenols and it was found that the best sorption properties were for the adsorbents with a proportion of 50:50. Therefore, the researchers demonstrated that both polarity and specific surface area were equally important. Marce and co-authors synthesized a series of hydrophilic sorbents by copolymerization of a polar monomer and a crosslinking agent (DVB). These sorbents were based on 4-vinylpyridine–divinylbenzene (4VP–DVB) [16], 4-vinylimidazole–divinylbenzene (4VIm–DVB) and N-vinylimidazole–divinylbenzene (NVIm–DVB) [17,18]. The specific surface area of these resins ranges from several to 700 m2 /g. When these resins were tested as sorbents for solidphase extraction for extracting polar compounds, the best results were obtained for NVIm–DVB, which has both the highest specific area and the quite high nitrogen content. Thus, both the proportion of polar functional groups and specific surface area are important for the retention of polar compounds [12]. Another approach of introducing polarity into a sorbent is the chemically modification of hydrophobic polymer skeleton with a suitable polar moiety. Fritz et al. first introduced this method by modifying resins with sulfonic, acetyl and hydroxymethyl groups [19,20]. More recently, Salih et al. modified a hydrophobic poly (styrene–divinylbenzene) with amine, nitrile and carboxyl groups [21], but the specific surface area of these polar polymeric sorbents are not very high. Therefore, in view of the above, an ideal material for adsorption of phenolic compounds would be a resin which has both polarity and large special surface area. Polymerization techniques to prepare macroporous polymer particles with large specific surface areas have come through three generations. The first-generation technique was prepared by divinylbenzene (DVB) copolymerization in the presence of the inert solvent. The specific surface area of the resulting product was often limited by the purity of DVB and the mechanism of the pore formation induced by phase separation in the process of polymerization [22]. The second-generation technique, called hypercrosslinked resin, was first introduced in the 1970s by Davankov et al. [23,24] and developed further by Jerabek and co-authors [25,26]. The hypercrosslinked resins were obtained by an extensive post-crosslinking of a preformed linear polystyrene or low-crosslinked polystyrene using a bifunctional crosslinking agent and a Friedel-Crafts catalyst. In this way, methylene bridges which reinforced the structure of the polymer matrix were formed between polymer chains. Alternatively, it is possible to use divinylbenzene–vinylbenzyl chloride (DVB–VBC) copolymer as the precursor in the presence of Friedel-Crafts catalyst and solvent [27,28]. This kind of hypercrosslinked resin exhibits an extremely high specific surface area and excellent sorption properties. However, the preparation of hypercrosslinked adsorbents is faced with a number of problems; the most serious one is the utilization of recognized carcinogen, chloromethyl methyl ether, as additional crosslinker in post-crosslinking reaction [29], and the production cost of the monomer vinylbenzyl chloride is rather high. The third-generation technique was the objective of the present research of this paper, just by post-crosslinking of the pendant vinyl groups on the starting macroporous styrene–divinylbenzene copolymers. The method of synthesis the post-crosslinked polymeric adsorbent by residual double bonds was first proposed by Ando et al. [30]. The specific surface area of the macroporous polymeric materials was great increased by Friedel-Crafts catalyzed reaction of the residual double bonds with neighboring benzene rings without an additional crosslinking agent. Recently, Yan’s group [31] and Jerabek’s group [29] investigated the post-crosslinking of the starting macroporous styrene–divinylbenzene copolymers via pendant vinyl groups using anhydrous ferric chloride as catalyst. Hao et al.
have synthesized the uniform micrometer-sized polydivinylbenzene microspheres with high surface areas via the Friedel-Crafts catalyst of anhydrous aluminum chloride [32]. However, the precursors of synthesizing such post-crosslinked adsorbents were non-polar, and their extreme hydrophobic surface was disadvantageous in industrial applications, such as extraction of polar compounds. The post-crosslinking of polar sorbents has been rarely recorded in the literature, despite the fact that such materials may be of more generic value in industry. Recently, we developed the technique of post-crosslinking reaction of pendant vinyl groups to prepare a novel polar polymeric adsorbent [33]. The resultant polymer particles have excellent adsorption capacity of phenol aqueous solution because the adsorbent possesses high special surface area and polarity. However, the relationship of polarity and special surface area has not been investigated in our previous work. In the present work, in order to study the effect of post-crosslinking reaction of pendant vinyl groups on the precursor polymer particles, we prepared a series of MMA-co-DVB and EGDMA-co-DVB adsorbents by using different contents of MMA and EGDMA. In the case of the N2 adsorption experiments the surface area, pore volume and average pore diameter were calculated from the adsorption branch of the isotherm. Phenol was selected as a model phenolic compound for further adsorption study. 2. Experimental 2.1. Materials Divinylbenzene (DVB 80% grade), methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) were kindly supplied by Nankai Hecheng S&T (Tianjin, China). All of them were washed with 10% aqueous sodium hydroxide and water and then dried over anhydrous magnesium sulfate prior to use. The initiators 2,2 azobisisobutyronitrile (AIBN) was supplied by Concord Technology (Tianjin, China) and was recrystallized from methanol. Poly(vinyl alcohol) (PVA) (88% hydrolyzed, Mn ∼125 000), anhydrous ferric chloride (FeCl3 ), 1,2-dichloroethane (DCE), toluene, heptane, acetone, methanol, phenol, sodium chloride were purchased from Tianjin Chemical Co. (Tianjin, China) and were used without further purification. The water used in the present work was deionized water without pH adjustment. 2.2. Preparation of the precursor copolymers The precursor copolymer beads were obtained by the usual suspension polymerization method in a 1000 mL three-necked round-bottomed flask reactor fitted with a mechanical stirrer, a reflux condenser and a thermometer [16]. At room temperature, the flask was charged with an aqueous phase containing 1% PVA and 10% NaCl. The organic phase, a mixture of MMA or EGDMA, DVB, the porogen, and the initiator (1.0 mol% in relation to the monomer mixture), was added to the aqueous phase. Suitable stirring rate was adjusted, and the polymerization process was performed at 353 K for 12 h. The composition of monomer mixtures is shown in Table 1. The prepared copolymer beads were collected, washed and extracted with acetone in Soxhlet apparatus for 10 h and then dried under vacuum at 333 K for 8 h. The sieving copolymer beads with fraction in the range 75–150 m were selected for further studies. 2.3. Post-crosslinking of precursor beads Scheme 1 presented the elaboration process of preparation the post-crosslinked adsorbent. The post-crosslinking reaction was carried out in 1,2-dichloroethane, using FeCl3 as Friedel-Crafts catalyst in amount corresponding to 20% of the starting copolymer
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Table 1 Polymerization conditions and typical properties of MMA-co-DVB and EGDMA-co-DVB copolymersa . Adsorbent
Compositions (mol/mol)
BET surface area (m2 /g)b
Pore volume (mL/g)c
Average pore diameter (nm)d
MMA/DVBe PDVB PDVBpc PDM-10 PDM-10pc PDM-20 PDM-20pc PDM-40 PDM-40pc
0/100 0/100 10/90 10/90 20/80 20/80 40/60 40/60
859.2 1128.6 762.6 988.4 638.6 821.8 582.8 601.5
1.61 1.72 1.42 1.59 1.08 1.32 1.06 1.10
10.03 9.33 10.85 9.07 9.40 8.63 9.87 9.82
EGDMA/DVB PDE-5 PDE-5pc PDE-10 PDE-10pc PDE-20 PDE-20pc
5/95 5/95 10/90 10/90 20/80 20/80
810.8 1018.2 714.7 811.5 698.8 716.2
1.51 1.66 1.34 1.46 1.49 1.53
10.62 9.76 11.73 10.47 11.29 11.16
Water uptake (g/g)
Swelling ratio
Water
Toluene
Methanol
1.58 2.03 1.64 1.98 1.71 1.85 1.92 1.89
1.19 1.06 1.25 1.10 1.29 1.11 1.36 1.32
1.42 1.23 1.51 1.26 1.56 1.31 1.70 1.65
1.26 1.15 1.29 1.16 1.32 1.12 1.41 1.38
1.72 2.16 1.77 1.93 2.02 1.97
1.32 1.12 1.27 1.13 1.35 1.31
1.46 1.19 1.50 1.24 1.49 1.42
1.23 1.06 1.31 1.11 1.28 1.22
a The organic phase/aqueous phase ratio was 1/4; the organic phase consisted of a monomer mixture (DVB and MMA/EGDMA), a porogen mixture (toluene and heptane), at a 1:2 ratio (v/v); the porogen toluene and heptane at a 2:1 ratio (v/v). b BET surface area for repeat polymer batches was reproducible within (5%). c BJH cumulative pore volume for pores between 1.7 and 300 nm in diameter. d BJH average pore diameter. e 80% grade monomer.
weight [29]. The reaction temperature was 353 K and the reaction time was 8 h. The copolymer beads after reaction were washed with acetone, acetone/0.1 M HCl (1:1 v/v), and water and then dried under vacuum at 333 K for 8 h. 2.4. Adsorption experiments of adsorbents The static adsorption of phenol was performed at the temperature of 298 K. Batch adsorption experiments were performed as follows: 0.100 g of dry adsorbent was introduced into a 100 mL conical flask, it was firstly wetted with 1.0 mL of methanol and then rinsed three times with deionized water. 25 mL of phenol aqueous solution containing a known concentration was added into each flask. The flasks were sealed and then transferred to an incubator shaker with thermostat (DSHZ-300, Taicang Laboratorial Equipment Factory, Jiangsu, China) and shaken under 120 rpm for 12 h at desired temperature to ensure that the adsorption process reached equilibrium. The equilibrium adsorption capacity Qe (mg/g) was calculated with Eq. (1): Qe =
V (C0 − Ce ) W
(1)
where V is the volume of solution (L), W is the mass of dry adsorbent (g), C0 and Ce (mg/L) denote the initial and equilibrium concentration of phenol in aqueous solution, respectively. Dynamic adsorption was carried out with a glass column (8 mm diameter and 200 mm length) packed with 5.0 mL (wet volume) post-crosslinked resin PDE-5pc at ambient temperature. Phenol aqueous solution of
3000 mg/L was passed through the column at a flow rate of 4.0 BV/h (BV is bed volume) until the adsorption curve completed, then industrial alcohol was used for desorbing resin PDE-5pc adsorbed phenol at a flow of 2.0 BV/h followed by 1.0 BV of deionized water. Breakthrough adsorption capacity and the total capacity were calculated based on the total amount of phenol removed when the concentration of the effluent from the column reached 5% and nearly 100% of the initial concentration, respectively. 2.5. Characterization The concentrations of phenol solutions were determined by using a UV-spectrophotometer (Purkinje General, Beijing, China) at the wavelength of 270 nm. The swelling ratio in water (toluene, methanol) defined as the ratio of the volume occupied by the resin when swollen in water (toluene, methanol) to its volume in the dry state at room temperature, was determined using a small measuring cylinder. Water uptake of the adsorbents was measured by the centrifugation method [13]. The specific surface area and the pore size distribution of the adsorbents were calculated, respectively by BET and BJH methods via the nitrogen adsorption and desorption curves at 77 K using a Micromeritics Tristar 3000 automatic surface area and porosity analyzer (Micromeritics Instrument Corp., USA). Before the BET surface area measurement, the adsorbents were outgassed at 333 K for 24 h on the degas port of the analyzer. Infrared spectra of the polymeric adsorbent before and after the post-crosslinking reaction were obtained from a MAGNA-560 FT-IR spectrometer (Nicolet, USA) with a pellet of powdered potas-
Scheme 1. Conceptual illustration of preparation of the post-crosslinked adsorbents.
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sium bromide and adsorbent in the range of 500–4000 cm−1 . The surface morphology of the copolymer particles was investigated by scanning electron microscopy (SEM) using a Scanning Electron Microscope (Hitachi 3500N). 3. Results and discussion 3.1. FT-IR spectra of precursor and post-crosslinked particles The FT-IR spectra of precursors and post-crosslinked polymeric resins MMA-co-DVB are shown in Fig. 1. Clearly, except for polymers PDVB and PDVBpc, all the polymers have a strong band with frequency at 1732 cm−1 corresponding to the strong stretching vibration of ester groups. From Fig. 1, it can be seen that the two characteristic bands for residual vinyl groups (CH CH2 ) at 1630 and 990 cm−1 almost disappeared after post-crosslinking reaction, and the peak at 902 cm−1 is also weakened. The phenomenon was in good agreement with other reports [29,34,35], which demonstrated that residual vinyl groups in starting polymeric resins were consumed during the reaction with the help of anhydrous ferric chloride. Fig. 1 illustrates that the content of MMA exerts strong influence on the amount of pendant vinyl groups onto starting copolymers. In present work, in order to probe that issue, we obtained four types of starting copolymer beads with a progressively increasing MMA concentration in monomer mixtures. As a result, increasing the proportion of MMA in monomer mixtures notably reduces the amount of the pendant vinyl groups onto the matrix of the starting copolymers. Furthermore, the residual vinyl groups of starting copolymers were almost absent when the content of MMA up to 40 mol% in the monomer mixtures, which may be attributed to the crosslinking density of monomer mixtures and the reactivity coefficients of monomers MMA and DVB. Sherrington co-authors [36] have reported that the level of residual unreacted vinyl groups of each resin could be determined by the degree of crosslinking, which was measured via using solid state 13 C nuclear magnetic resonance (NMR) techniques. As in the previous case, decreasing the amount of the crosslinking monomer (DVB) reduces that of the residual vinyl groups. The reactivity coefficients (rMMA and rDVB ) reflect the inherent tendencies of a radical to react with its own monomer relative to the comonomer. The reactivity coefficients of MMA (rMMA ) and DVB (rDVB ) are less than 1 at various temperatures [37], which indicate the monomer is more likely to copolymerize with comonomer. He and co-authors [37] and Cheng et al. [38] have extensively studied that the reaction kinetics of pendant vinyl groups in the divinylbenzene suspension polymer-
Fig. 1. FT-IR spectra of MMA-co-DVB adsorbents before and after post-crosslinking reaction.
Fig. 2. FT-IR spectra of EGDMA-co-DVB adsorbents before and after postcrosslinking reaction.
ization. They believed that the activity of the second double bonds of the crosslinker DVB was rather low. Therefore, a lot of pendant vinyl groups remained on the polymer after the polymerization was completed. After a polar monomer MMA was added into the DVB polymerization system, more second double bonds would be consumed in the process of copolymerization reaction. Thereby, the residual vinyl groups of precursor resins would diminish correspondingly. For the purpose of improving the mechanical rigidity and special surface area of resins, a series of polar crosslinking monomers such as EGDMA were used to replace MMA in the polymerization mixture. Despite the copolymerization of monomer mixtures EGDMA and DVB was reported in a lot of literature [39,40], the study of pendant vinyl groups on the skeleton of resins did not seem to be recorded. Thereby an investigation was carried out in the way which was employed in the study of the MMA-co-DVB adsorbents. The results of FT-IR spectra of precursors and post-crosslinked polymeric resins EGDMA-co-DVB are summarized in Fig. 2. As can be seen from Figs. 1 and 2, the IR spectra of copolymers EGDMA-coDVB and MMA-co-DVB are almost identical, which indicate the precursors of EGDMA-co-DVB possess residual vinyl groups and they disappeared after post-crosslinking reaction as well. 3.2. Resins porosity characteristics Table 1 lists some important characteristics of MMA-co-DVB resins. The resins were prepared by using technical DVB and MMA. After post-crosslinking modification, both the specific surface area and pore volume of the precursor resins PDVB, PDM-10, PDM20 showed a remarkable increase. We can also discover that the increase of specific surface area is more significant for precursor resins with higher DVB content than with lower DVB content. That is because the residual vinyl group content of the precursor resins decreases with the decrease of the DVB content, which leads to less new crosslinking bonds created by the Friedel-Crafts reaction. However, those phenomena are not obvious in the precursor resin PDM-40, which can be attributed to the fact that there are almost no pendant vinyl groups onto PDM-40. Similar results were reported by Nyhus et al. [41] and Yan and co-authors [42]. Resins EGDMAco-DVB were prepared by the copolymerization of EGDMA and DVB, and some typical properties are presented in Table 1. Similarly, both the surface area and the pore volume of these starting polymeric resins increase after post-crosslinking reaction. Nevertheless, when the content of EGDMA is 20 mol%, there is almost no increase in special surface area and pore volume. Fig. 3(a) and (b) shows the specific surface area of copolymers before and after
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Fig. 3. Dependence of the specific surface area of precursors and post-crosslinked copolymers reaction on the ratio of polar monomer to divinylbenzene (a) MMA/DVB and (b) EGDMA/DVB. Specific surface area: (A) before post-crosslinking reaction Sb ; (B) after post-crosslinking reaction Sa ; (C) Sa −Sb .
the post-crosslinking reaction has correlation with the ratio of polar monomer MMA or EGDMA to DVB. Fig. 3 indicates that the addition of specific surface area declines with the decrease of DVB content. A much better representation of the pore structure in resins is of course given by the pore size distribution. The pore size distribution curves for the copolymers PDM-10 and PDE-10 prior to and after post-crosslinking reaction as revealed by the BJH (Barret, Joyner, and Halenda) method [43] are shown in Fig. 4(a) and (b). Compared with the precursors PDM-10 and PDE-10, post-crosslinked copolymers PDM-10pc and PDE-10pc presented more micropores after post-crosslinking reaction. The result that the average pore diameter of precursors in Table 1 decreased after post-crosslinking modification could be attributed to the increase of crosslinking degree. One of the most prominent properties of precursor and postcrosslinked resins is their response to solvents such as water, toluene and methanol, and which is demonstrated by the swelling ratio of solvents in Table 1. Moreover it shows the changing characteristic of precursor resins networks after post-crosslinking reaction. As is shown in Table 1, not only do the swelling ratio data for the thermodynamically compatible solvent, toluene, decrease relative to the precursor in the case of post-crosslinked resin, but also the two thermodynamically “poor” solvents, n-hexane and water, also exhibit a moderate decline. The phenomenon of different swelling ratio in various solvents demonstrates that the stability and rigidity of skeleton structure of the precursor resins increased after post-crosslinking reaction, which is significant for practical application. The water uptake of resins typically approaches 2 g/g. Before the water uptake of the resin was mea-
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Fig. 4. Pore size distribution curves of resins obtained by the nitrogen adsorption technique using BJH method; (a) precursor PDM-10 and its post-crosslinked derivative PDM-10pc (b) precursor PDE-10 and its post-crosslinked derivative PDE-10pc.
sured, it was progressively swollen by toluene, toluene/acetone, acetone, acetone/water and finally rinsed three times with deionized water [13]. (Resins do not swell directly in water, thus it is necessary to wash them in a solvent to increase some expansion of the polymeric network.) 3.3. Scanning electron microscopy (SEM) Scanning electron micrographs of two typical precursor resins (PDM-10 and PDE-10) and their post-crosslinking derivatives (PDM-10pc and PDE-10pc) are presented in Fig. 5. It is noteworthy that the precursors become more highly porous after the Friedel-Crafts modification. This surface morphology change could be fairly understandable if we consider that some of the newly formed crosslinking bonds may be “glued” together. The additional crosslinks increased the rigidity of the polymer beads, which indicated that the specific surface area of precursor resins enhanced significantly after post-crosslinking reaction. Similar results were reported elsewhere by Jerabek and co-authors [29] and Gong and co-authors [32]. 3.4. Adsorption isotherms Fig. 6(a)–(c) presented the equilibrium adsorption isotherms of phenol from aqueous solution onto copolymers MMA-co-DVB and EGDMA-co-DVB at the temperature of 298 K. Post-crosslinked adsorbents exhibited higher adsorption capacity than precursor adsorbents, which resulted from the specific surface area has great increase after post-crosslinking reaction.
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Fig. 5. Scanning electron micrographs of copolymer beads before and after Friedel-Crafts post-crosslinking: (A), PDM-10; (B), PDM-10pc; (C), PDE-10; (D), PDE-10pc. Magnification is 300× and 20k×.
The Freundlich equation and Langmuir equation were employed to fit the equilibrium adsorption data. The Freundlich model and Langmuir model can be expressed as Eq. (2) [44] and Eq. (3) [45–47], respectively. log Qe = log KF +
1 log Ce n
(2)
Ce 1 Ce = + KL Qm Qm Qe
(3)
where KF is a characteristic parameter as a measure of adsorption capacity ((mg/g)/(mg/L)1/n ), whereas n refers to as adsorption intensity and is an indicator of the favorableness of the adsorbent/adsorbate system. Qm is the maximum adsorption capacity (mg/g), and KL is a parameter which relates to the adsorption energy (L/g). The values of isotherm constants at 298 K are listed in Table 2. It is observed that the equilibrium data fitted both the Freundlich and Langmuir model well with correlation coefficient (R2 ) are larger than 0.99. The exponent (n) is larger than 1 in all cases, which demonstrates the adsorption of phenol onto adsorbents are favorable. Table 2 shows that PDE-5pc has better adsorption capacity than other adsorbents. The maximum adsorption capacity Qm of phenol onto PDT-5pc is 250.6 mg/g, which is larger than the results reported by other researchers and commercial polymeric adsorbents Amberlite XAD-4 and AB-8 [13,14,33]. It is commonly known that adsorption capacity of adsorbent from aqueous solution is dominated by many factors and the adsorbate–adsorbent interaction will play an important role. The Table 2 Freundlich and Langmuir isotherm parameters of phenol adsorption onto different adsorbents at 298 K. Adsorbent
PDVB PDVBpc PDM-10 PDM-10pc PDM-20 PDM-20pc PDM-40 PDM-40pc PDE-5 PDE-5pc PDE-10 PDE-10pc PDE-20 PDE-20pc
Freundlich model
Langmuir model 2
KF
n
R
KL
Qm
R2
3.394 6.937 3.811 7.507 3.506 5.490 3.802 4.422 4.047 8.326 3.608 5.477 3.986 4.815
2.069 2.245 2.116 2.478 2.104 2.468 2.192 2.385 2.132 2.438 2.103 2.451 2.211 2.408
0.995 0.999 0.995 0.998 0.996 0.998 0.999 0.997 0.998 0.999 0.998 0.999 0.997 0.998
1.424 1.515 1.429 1.716 1.432 2.115 1.268 1.730 1.301 1.587 1.395 1.967 1.306 1.608
190.1 231.5 194.2 228.1 185.2 197.6 183.8 184.9 202.3 250.6 193.0 212.4 185.5 187.3
0.999 0.996 0.994 0.996 0.992 0.995 0.998 0.997 0.998 0.999 0.995 0.998 0.996 0.999
effective interaction causing different adsorption behaviors of different adsorbates mainly includes: (1) hydrophobic interaction. The interactions with the adsorbates are basically by the – sites of the aromatic rings that make up the sorbent structure; (2) van der Waals force. It mainly includes dispersion interaction, dipole–dipole and dipole–induced dipole interactions; (3) hydrogen bonding interactions [12,13,48]. The adsorbing capacity of phenol for post-crosslinking derivatives is larger than precursors. This can be explained that the specific surface area of starting copolymer is greatly increased after post-crosslinking reaction. Accordingly, more – sites are available to interact with the adsorbate. Thus, the post-crosslinking derivatives possess of larger number of – sites which are helpful for adsorption of phenol than initial copolymers. Compared with the post-crosslinked resin PDE-5pc, the adsorbent PDVBpc has larger specific surface area, but its adsorption capacity for phenol on PDVBpc is lower than PDE-5pc, which further demonstrates that besides specific surface area, adsorption capacity would be affected by other factors. The surface of PDE-5pc resin contains polar groups of ester groups, and the polarity matching between the PDE-5pc resin and phenol may partially account for the increase in the adsorption capacities [13,18]. Besides the difference in polarity, the hydrogen bonding should also be taken into account, which occurs between the carbonyl group on PDE5pc and the hydroxyl group of phenol, and plays an important role in the adsorption of phenol [49]. For the precursors PDM-20, PDM-40, PDE-10, PDE-20, post-crosslinked adsorbents PDM-20pc, PDM-40pc, PDE-10pc, PDE-20pc, they have lower specific surface area then adsorbents PDVBpc, PDM-10pc, PDE-5pc, although the formation of hydrogen bonding could increase the adsorption capacity of PDM-20, PDM-40, PDE-10, PDE-20, PDM-20pc, PDM40pc, PDE-10pc, PDE-20pc, it cannot compensate for the decrease in adsorption capacity induced by the loss of the specific surface area. In a word, adsorption capacity could be affected by several factors, including specific surface area, adsorbent polarity and pore structure. 3.5. Dynamic adsorption and desorption Because of the satisfactory adsorption capacity and affinity for phenol onto the post-crosslinked copolymer PDE-5pc, it is hopeful that PDE-5pc will be developed as a polymeric adsorbent for the removal and recovery of phenolic contaminants from drinking water or industrial effluent. It is necessary to test the dynamic adsorption and desorption. The results of glass column dynamic adsorption of phenol on PDE-5pc are shown in Fig. 7, where CV is
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Fig. 7. Adsorption and desorption dynamic curves of phenol onto PDE-5pc at room temperature.
4. Conclusions A series of novel post-crosslinked polar polymeric adsorbents were prepared to enhance adsorption of phenol from aqueous solution. Both the specific surface area and the pore volume of polymeric adsorbents increased remarkably after post-crosslinking by Friedel-Crafts reaction of pendant vinyl groups onto precursor copolymers. The adsorbent PDE-5pc exhibited the highest adsorption capacity of phenol, which was traceable to its large specific surface area as well as the polar groups on its network. The adsorption isotherm data of all adsorbents were in good agreement with the Freundlich model and Langmuir model. On the basis of those results, further work on the syntheses of adsorbents with higher specific surface areas and higher polarity is planned. Acknowledgment The authors gratefully acknowledge the Tianjin Municipal Science and Technology Commission (Grant no. 09JCYBJC13600). References
Fig. 6. Comparison of adsorption isotherms of phenol (a) onto PDVB and PDVBpc at 298 K; (b) onto copolymers MMA-co-DVB at 298 K; (c) onto copolymers EGDMAco-DVB at 298 K.
the concentration at different bed volumes of the effluent (mg/L), CI is the concentration of initial solution (mg/L), and CR is the concentration at different bed volumes of regenerative reagent industrial alcohol (g/L). The results of breakthrough adsorption capacity and the total adsorption capacity are 56.8 and 60.1 mg/mL wet resin, respectively. As can be seen from Fig. 7, the shape of the adsorption curve is very sharp indicating that the adsorption of phenol onto PDE-5pc reached equilibrium quickly after leakage, which is crucial for industrial applications. Industrial alcohol was used to desorb phenol from PDE-5pc resin column. Nearly 100% regeneration efficiency was achieved by 3.0 BV industrial ethanol at room temperature (Fig. 7).
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