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Preparation of polymeric microspheres for removal of boron by means of sorption-membrane filtration hybrid
Desalination 283 (2011) 193–197
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation of polymeric microspheres for removal of boron by means of sorption-membrane filtration hybrid Joanna Wolska ⁎, Marek Bryjak Division of Polymer and Carbonaceous Materials, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50–370 Wroclaw, Poland
a r t i c l e
i n f o
Article history: Received 23 October 2010 Received in revised form 21 February 2011 Accepted 22 February 2011 Available online 29 March 2011 Keywords: Membrane emulsification Polymer microspheres Chemical modification Microwave modification Boron sorption
a b s t r a c t Three types of polymeric microspheres with different amounts of vinylbenzyl chloride-styrene-divinylbenzene monomers were prepared. The synthesized particles were obtained by membrane emulsification followed by suspension polymerization. Optimization of emulsification and polymerization parameters allowed to obtain microspheres with 30 μm diameter and narrow size distribution. Synthesized microspheres were modified with N-methyl-D-glucamine at reflux and in a microwave reactor. The resins were tested in sorption of boron from aqueous solutions and compared to commercial materials. It was noted that resin obtained in microwave modification showed the best sorption properties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction When World Health Organization modified the lower permissible level for boron in potable water to 2.4 mg/L its content in irrigation water was kept as low as 0.5 mg/L [1]. The effect of boron's negative influence on plant metabolism is when at higher concentration it affects significantly the plant growth and results in ‘boron poisoning’ [2]. In such circumstances, boron removal from water still remains a critical issue for agriculture. There is no easy method for boron depletion and suitable technologies are progressively developed in various places. Some of them are listed in the following reviews [3–5]. Lately, the hybrid processes have been investigated intensively. They combine sorption on a boron selective resin (BSR) with membrane filtration [6,7]. Hybrid processes reveal many advantages in comparison to the fixed bead systems [8]. The only one their disadvantage is a shortage of available fine BSR particles. The market offers materials with too large diameter to be useful in hybrid systems directly. For this reason, some authors tried to apply the ground sorbents despite some doubts of their diameter stability were raised [9]. The shortage of fine powdered adsorbents forced us to find the useful method for synthesis of such microspheres. Microspheres with a narrow distribution of particle size are used in medicine, pharmacology, in diagnostic and analytic devices, in paint industry, in food, cosmetics and in electronics. They can be used as carriers for catalysts, as chromatographic packing and as ion ex-
⁎ Corresponding author. Tel.: +48 71 320 23 83; fax: +48 71 320 29 87. E-mail address: [email protected] (J. Wolska). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.02.052
changers. As microspheres the particles with sizes from 0.1 to dozens of microns, are made up from organic or inorganic matrices [10]. Due to several routes for microspheres synthesis, it is possible to obtain particles with different structure, morphology, shape or different interior and surface layers [10,11]. Microspheres can be obtained from the synthetic and natural polymers, using crystallization, solvent extraction, phase separation or solvent evaporation from the droplets of the polymer solution [12]. They are obtained also directly during the polymerization process [10,11,13] in emulsion [14], microemulsion [15] or suspension [16], as well as by interpolymerization [17], encapsulation [18] or seeded polymerization [19]. Recently, a two-step synthesis method has received the growing popularity. It compromises monomer emulsification and polymerization of obtained droplets. In comparison to the conventional suspension polymerization, it offers possibility to obtain monodisperse beads with high production yield [20]. The goal of this study was to find a synthetic route to obtain fine particles of polymer sorbent, with diameter in the range of 20–30 μm and low dimension polydispersivity, bearing N-methyl-D-glucamine functionalities.
2. Experimental 2.1. Chemical reagents All reagents were purchased from Sigma-Aldrich Co. Monomers were distilled prior to their use. Other reagents were used as received. Solvents were purchased from POCh Gliwice, Poland, and use without any purification.
194
J. Wolska, M. Bryjak / Desalination 283 (2011) 193–197
2.2. Polymerization and modification reactions 2.2.1. Synthesis of VBC/S/DVB terpolymer VBC/S/DVB matrices were prepared from vinylbenzyl chloride (VBC), styrene (S) and divinylbenzene (DVB) using membrane emulsification followed by suspension polymerization. The obtained polymers had 6 wt.% of crosslinker agent with the following ratio of VBC to S 5:5 (called as Polymer A), 6:4 (Polymer B) and 7:3 (Polymer C). Polymerization was initiated by azoizobutyronitrile (1% wt). More details on the synthesis procedure can be found elsewhere [21]. Membrane emulsification process was carried in a unit supplied by Micropore Ltd, equipped with metal membrane [22] with regularly arranged pores of 5 μm size. The droplets of monomers were polymerized according to the suspension polymerization protocol, at the following temperature regimes: 60 °C–2 h, 85 °C–12 h. Polymerized microspheres were rinsed with hot water, cold water and acetone, dried and extracted with hot toluene in a Soxhlet apparatus. 2.2.2. Preparation of the resins The polymeric resins were prepared by modification of polymer matrices at reflux (BP) and in a microwave reactor (MW). In the first case, the VBC/S/DVB spheres were swollen in dioxane, DIO, and reacted with N-methyl-D-glucamine, NMDG. The reaction was carried out in 1:1 mixture of dioxane:water, with 10 times molar excess of NMDG in respect to chloride content. The reaction was carried out for 2 h. After modification, resin was washed with ethanol, ethanol/water (1:1) and water. Finally, spheres were placed in a column and washed with 1 M HCl and 1 M NaOH aqueous solutions. For modification in a microwave reactor, the spheres were swollen in solution of NMDG in dimethyl sulfoxide, DMSO, or dimethyl formamide, DMF. The same excess of amine to chloride groups as in BP route was set. The modification was carried out in the microwave reactor for 10 min at energy of 100 W. After that time the resins were washed with ethanol, ethanol/water (1:1) and water, and rinsed with 1 M HCl and 1 M NaOH solutions. 2.2.3. Analyses The morphology of the dispersed phase was investigated by SEM microscope. The average diameters and size distribution of microspheres were measured using a Mastersizer X (Melvern Instruments GmbH, Germany). Water regain was measured using centrifugation method, by placing 0.5 g of swollen polymer in a fritted-glass column and centrifuging it for 5 min with 3000 rpm. The sample was weighted, and dried at 105 °C for 24 h. After cooling to room temperature the sample was weighted again. Water regain was calculated as (mw − md)/md, where mw is the weight of swollen polymer after centrifugation, and md is the dry weight of polymer [23]. Chloride content in polymers was measured by burning about 20 mg of dry sample in a flask containing 3% hydrogen peroxide solution. The content of chloride was determined by the Volhardt's method [24]. Nitrogen content in the resins was determinate by the Kjeldahl method after mineralization of the sample of resin (about 200 mg) in concentrated sulfuric acid containing copper sulfate and potassium sulfate [25]. FTIR spectra of resins were recorded on a Perkin Elmer Sys. 2000 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was analyzed on a SPES ESCA system equipped with a PHOIBOS 100 analyzer with X-ray generated by Mg anode at 10 kV and 200 W. The takeoff angle was set as 90o. Pressure in the analysis chamber was kept at 5 10− 9 mbar level and pass energy was set at 30 eV. 2.2.4. Evaluation of sorption properties The sorption capacity of resins towards boron (B) was determined by contacting the samples with 20 mL of B solution (2 mg/L). Boron
solution was prepared with MiliQ water, and re-crystallized boric acid. After 24 h, the samples were filtered and concentration of B was determined. The boron concentration was detected by use of Curcumine method (λmax = 543 nm) [26]. Jasco V-530 spectrophotometer was applied. The range of boron standards was kept between 0.1 and 2.0 mg B/L. Boron sorption per gram of dry resin (q) and distribution coefficient (K) were calculated. 2.2.5. Sorption isotherms For the determination of sorption isotherms, different amounts of samples (from 0.125 to 1.75 g dry resin/L solution) were shaken with 50 mL boon solution (2 mg/L of B) at room temperature for 24 h. After that time the resin was filtered and concentration of boron was determinate. The maximum boron uptake (qmax) was calculated according to Langmuir equation. 2.2.6. Sorption kinetics Sorption kinetics were determined for selected resins. Typical procedure was as follows: a flask with a capacity of 250 mL was filled with resin to give a suspension of 0.5 g dry polymer/L. Boron solution (2 mg/L, 200 mL) was added to the flask and mixed gently. The samples of solution were taken out at different time to determine boron concentration. 3. Results and discussion 3.1. Preparation of chelating resins Preparation of monodisperse microspheres having a relatively small size is one of the important tasks for materials engineering. Using membrane emulsification of monomers mixture and polymerization of emulsion one is able to obtain polymer beads with regular shape (see Figs. 1, 2) and very narrow size distribution. Using this method three types of polymer microspheres were synthesized. They had gel-like structure and contained various amount of VBC as is described in Table 1. A high degree of crosslinking was needed to get the mechanically resistant polymer beads while the lack of neutral solvents guaranteed their solid structure (see the example in Fig. 1). It was noted that the average diameter of microspheres was 30 μm and was about 6 times larger than the diameter of pores in the membrane. The obtained spheres were characterized by their size and polydispersity, water regain and chloride content. These properties are listed in Table 1 also. Analysis of chloride content showed that sphere A have 2.30 mmol/g, sphere B — 3.45 mmol/g and sphere C — 3.60 mmol/g of chloride. These values correspond to 76%, 85% and 86% yield of VBC incorporation to the polymer matrix, respectively. As chloride content in samples B and C was almost the same, it was decided to not increase more vinylbenzyl chloride concentration in the monomer mixture.
Fig. 1. SEM picture of polymer B.
J. Wolska, M. Bryjak / Desalination 283 (2011) 193–197
195
Trans. [%]
a b c
4000
3000
2000
1000
0
Wavelength [cm-1] Fig. 3. FTIR spectra for: unmodified polymer C (a), resin C/BP (b), resin C/DMSO/MW (c). Fig. 2. SEM picture of polymer C.
Water regain for all evaluated matrices was as small as 11%. It confirmed the gel structure of the prepared spheres. Modification of obtained polymers was carried out by reacting NMDG with methyl chloride groups of VBC. Two methods of modification were employed: i) at reflux, and ii) in a microwave reactor. The properties of obtained resins are summarized in Table 2. Depending on the used method, a different degree of substitution of chloromethyl groups was observed. The best method seems to be modification in boiling point. However, for DMSO as solvent the microwave modification offers similar results. The water regain of obtained resins was related to used method and the kind of polymer matrix. The resins with high content of nitrogen were more hydrophilic and had water regain on the range of 0.4–0.59 g/g. It looks that B and C matrices are the best materials for preparation of NMDG substituted resins. The chemical structure of C resin, before and after its modification, was investigated by FTIR method. In Fig. 3a, FTIR spectrum of unmodified matrix C is shown. Polymers having methyl chloride groups gave an absorption band at 1265 cm− 1 that is attributed to C\Cl bond [27,28]. The peak disappeared when \Cl was substituted by NMDG (Fig. 3b and c). When the modification developed the changes were noted in the 2790–700 cm− 1 region. On each spectrum, valence band of \CH was detected at the 2920–2850 cm− 1 region. As a result of chloride substitution by NMDG, this band became stronger. In the case
Table 1 Characteristic of polymers VBC/S/DVB. Polymer
VBC:S ratio (wt.:wt.)
Chloride content [mmol/g]
Average diameter [μm]
SPAN
A B C
5:5 6:4 7:3
2.30 3.45 3.66
28 26 30
0.8 0.9 1.0
of C/BP resin (Fig. 3b), it was no band at 1265 cm− 1. It confirmed the complete replacement of chloride by N-methyl-D-glucamine. Survey XPS scans allowed to estimate the surface atom composition of the studied materials. The results are presented in Table 3. Following the data, one notes relatively high concentration of nitrogen for C/BP and C/DMSO/MW resins. It means that these samples contained the highest contents of N-methyl-D-glucamine functionalities. What is more, the NMDG substitution was completed in these samples as no Cl2p peak was found in the spectra. 3.2. Evaluation of the sorption properties The sorption properties of obtained resins were checked to evaluate their usefulness for the hybrid systems. To determine it, a boric acid stock solution was prepared. It contained 2 mg B/L in H2O and its pH value was adjusted to 8.2. The obtained results, presented as distribution coefficient K = (Co − Ce)/Ce where Co and Ce are boron concentration before and after sorption, are shown in Table 4. It is seen that the resin obtained from polymer C, has the strongest affinity towards boric acid. The highest distribution coefficient (K) was noted for two its types: C/DMSO/MW and C/BP. 3.3. Sorption isotherms and kinetics studies The above presented studies forced us to investigate the sorption process more deeply. Fig. 4 shows sorption isotherms of resins at room temperature. As expected all studied sorbents showed Langmuir-shape relationship. For this reason Langmuir equation was applied to calculate the sorbent capacity value (qmax):
q=
qmax ⋅ c a+c
ð1Þ
where q is the boron uptake (mmol/gresin), c is the equilibrium concentration (mmol/L), and a is a constant (mmol/L). The values of qmax calculated for investigated resins are listed in Table 5.
Table 2 Characteristics of polymeric resins. Resin
Matrix
Solvent
Method of modification
Nitrogen content [mmol/g]
Chloride content [mmol/g]
Water regain [g/g]
A/BP A/DMSO/MW A/DMF/MW B/BP B/DMSO/MW B/DMF/MW C/BP C/DMSO/MW C/DMF/MW
A
DIO DMSO DMF DIO DMSO DMF DIO DMSO DMF
BP MW MW BP MW MW PB MW MW
1.77 0.88 0.32 2.08 1.16 0.47 2.12 2.05 0.45
0 1.21 1.51 0 0.80 1.90 0 0.03 2.70
0.55 0.26 0.18 0.51 0.38 0.21 0.55 0.46 0.19
B
C
Table 3 XPS surface composition in VBC/S/DVB matrix and resins. Sample
B C B/BP B/DMF/MW B/DMSO/MW C/BP C/DMSO/MW
Sample composition [%] C1s
O1s
Cl2p
N1s
75.7 76.8 69.9 69.8 71.5 75.4 78.3
23.8 23.4 28.8 28.8 26.7 22.5 19.0
0.5 0.8 – 0.3 0.1 – –
– – 1.3 1.1 1.7 2.1 2.7
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J. Wolska, M. Bryjak / Desalination 283 (2011) 193–197 Table 4 Sorption of boron by resins from 2 mg/L boron solutions.
Table 5 Maximum uptake of boron by evaluated resins.
Resin
Distribution coefficient, K
Resin
qmax [mmol/gresin]
A/BP A/DMSO/MW A/DMF/MW B/BP B/DMSO/MW B/DMF/MW C/BP C/DMSO/MW C/DMF/MW
15.6 9.9 1.5 11.7 8.2 0.11 97 145 1.5
A/BP A/DMSO/MW A/DMF/MW B/BP B/DMSO/MW B/DMF/MW C/BP C/DMSO/MW C/DMF/MW
0.25 – – 0.26 0.21 – 1.04 1.06 –
For both sorbents, a slight decrease in sorption properties was observed after the first cycle. The sorption of boron was reduced to 96% for C/DMSO/MW and to 93% for C/BP of basic value and remained constant for the next cycles. It is noteworthy that after the fifth cycle, sorption of boron was still high: the C/DMSO/MW resin was able to adsorb 1.0 mmol B/g while the C/BP resin — 0.93 mmol B/g. These results confirm that NMDG ligands were covalently linked to the polymer matrix and polymer microspheres were resistant to the pH-shocks. 4. Conclusion Process of suspension formation by membrane emulsification followed by suspension polymerization offers particles with 30 μm diameter and narrow size distribution. Subsequent modification of prepared microspheres results in obtaining good sorbents for boron removal and these sorbents can be used in the hybrid systems. The 1,2 1 0,8
C/C0
At the first glance, both C/BP and C/DMSO/MW resins show similar sorption capacity as commercial sorbents. Roughly recalculation of the sorption capacity for Purolite S108, Diaion 2B, IRA-743 or Dowex XUS shows that this value is somewhere between 0.8 and 1.0 mmol/g. The more carefully check of data in Table 5 reveals that resins capacities are in relationship to nitrogen contents, that can be roughly identified as NMDG concentration. However, the direct comparison of qmax with nitrogen concentration (see Table 2) allows to conclude that not everyone ligand participate in sorption. Access to the internal functional groups was not easy due to the solid structure of polymer matrix. For C/DMSO/MW resin, the ratio of qmax to nitrogen content was 0.65 while for C/BP resin it reached smaller value. It means that ca. 50% of NMGD ligands participated in boron binding. The kinetics of boron sorption was the next property that was evaluated. The studies were conducted for the best materials: C/DMSO/MW and C/BP. Fig. 5 shows the percentages of boron uptake as a function of time. Sorption of boron by both studied resins was very fast. It can be seen, that C/DMSO/MW resin reduced boron concentration about twotimes faster than C/BP resin did. It is expected that this phenomenon is related to better accessibility of NMDG ligands for the first resin. Similar results were obtained when distribution coefficients are compared (see Table 4).
3.4. Resins stability in sorption/desorption cycles
0,6
C/M/DMSO
C/BP
0,4
Complete desorption of boron from resins is usually generated by 5% H2SO4 solution. After it, the resin should be equilibrated with solution having pH as high as 8.2. Such procedure exposes ionexchange resins to different pH conditions and may affect their sorption properties. In order to check the resins stability, five sorption/desorption cycles were conducted for both selected sorbents. After each cycle, the sorption capacity was evaluated. To make the comparison easy, sorption was normalized to the fresh resin (Fig. 6).
1 C/DMSO/M
0,8
C/BP B/BP
0,6
B/DMSO/M
0,4
A/BP A/DMSO/M
0,2
0 0
500
1000
1500
TIME [min] Fig. 5. Kinetics of sorption of boron on C/BP and C/DMSO/MW.
procent of initial max. uptake
boron uptake [mmol/g]
1,2
0,2
C/DMSO/M
100
C/BP
90 80 70 60 50 1
0 0
0,025
0,05
0,075
0,1
0,125
0,15
0,175
2
3
4
5
sorption/desorption cycle
equlibrium conc. [mmol/L] Fig. 4. Sorption isotherms of boron on obtained resins.
Fig. 6. Sorption properties of resins: C/DMSO/MW and C/BP in five cycles of sorption and desorption.
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best resin is obtained from 7:3 VBC:S monomer mixture when the polymer matrix is modified with NMDG in the microwave reactor. Short time of reaction and the best sorption properties for C/DMSO/ MW resin show the best way for resins preparation. Acknowledgement This work was financially supported by Wroclaw University of Technology grant No. 344045/Z0309. References [1] N. Kabay, E. Güler, M. Bryjak, Desalination 261 (2010) 212. [2] I. Yilmaz, N. Kabay, M. Yuksel, R. Holdich, M. Bryjak, Sep. Sci. Technol. 42 (2007) 1013. [3] N. Kabay, M. Bryjak, S. Schlosser, M. Kitis, S. Avlonitis, Z. Matejka, I. Al-Mutaz, M. Yuksel, Desalination 223 (2008) 38. [4] I. Yilmaz Ipek, R. Holdich, N. Kabay, M. Bryjak, M. Yuksel, React. Func. Polym. 67 (2007) 1628. [5] M. Bryjak, J. Wolska, I. Soroko, N. Kabay, Desalination 241 (2009) 127. [6] A. Koltuniewicz, A. Witek, K. Bezak, J. Membr. Sci. 239 (2004) 129. [7] N. Kabay, I. Yilmaz-Ipek, I. Soroko, M. Makowski, O. Kirmizisakal, S. Yag, M. Bryjak, M. Yuksel, Desalination 241 (2009) 167.
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation of polymeric microspheres for removal of boron by means of sorption-membrane filtration hybrid Joanna Wolska ⁎, Marek Bryjak Division of Polymer and Carbonaceous Materials, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50–370 Wroclaw, Poland
a r t i c l e
i n f o
Article history: Received 23 October 2010 Received in revised form 21 February 2011 Accepted 22 February 2011 Available online 29 March 2011 Keywords: Membrane emulsification Polymer microspheres Chemical modification Microwave modification Boron sorption
a b s t r a c t Three types of polymeric microspheres with different amounts of vinylbenzyl chloride-styrene-divinylbenzene monomers were prepared. The synthesized particles were obtained by membrane emulsification followed by suspension polymerization. Optimization of emulsification and polymerization parameters allowed to obtain microspheres with 30 μm diameter and narrow size distribution. Synthesized microspheres were modified with N-methyl-D-glucamine at reflux and in a microwave reactor. The resins were tested in sorption of boron from aqueous solutions and compared to commercial materials. It was noted that resin obtained in microwave modification showed the best sorption properties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction When World Health Organization modified the lower permissible level for boron in potable water to 2.4 mg/L its content in irrigation water was kept as low as 0.5 mg/L [1]. The effect of boron's negative influence on plant metabolism is when at higher concentration it affects significantly the plant growth and results in ‘boron poisoning’ [2]. In such circumstances, boron removal from water still remains a critical issue for agriculture. There is no easy method for boron depletion and suitable technologies are progressively developed in various places. Some of them are listed in the following reviews [3–5]. Lately, the hybrid processes have been investigated intensively. They combine sorption on a boron selective resin (BSR) with membrane filtration [6,7]. Hybrid processes reveal many advantages in comparison to the fixed bead systems [8]. The only one their disadvantage is a shortage of available fine BSR particles. The market offers materials with too large diameter to be useful in hybrid systems directly. For this reason, some authors tried to apply the ground sorbents despite some doubts of their diameter stability were raised [9]. The shortage of fine powdered adsorbents forced us to find the useful method for synthesis of such microspheres. Microspheres with a narrow distribution of particle size are used in medicine, pharmacology, in diagnostic and analytic devices, in paint industry, in food, cosmetics and in electronics. They can be used as carriers for catalysts, as chromatographic packing and as ion ex-
⁎ Corresponding author. Tel.: +48 71 320 23 83; fax: +48 71 320 29 87. E-mail address: [email protected] (J. Wolska). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.02.052
changers. As microspheres the particles with sizes from 0.1 to dozens of microns, are made up from organic or inorganic matrices [10]. Due to several routes for microspheres synthesis, it is possible to obtain particles with different structure, morphology, shape or different interior and surface layers [10,11]. Microspheres can be obtained from the synthetic and natural polymers, using crystallization, solvent extraction, phase separation or solvent evaporation from the droplets of the polymer solution [12]. They are obtained also directly during the polymerization process [10,11,13] in emulsion [14], microemulsion [15] or suspension [16], as well as by interpolymerization [17], encapsulation [18] or seeded polymerization [19]. Recently, a two-step synthesis method has received the growing popularity. It compromises monomer emulsification and polymerization of obtained droplets. In comparison to the conventional suspension polymerization, it offers possibility to obtain monodisperse beads with high production yield [20]. The goal of this study was to find a synthetic route to obtain fine particles of polymer sorbent, with diameter in the range of 20–30 μm and low dimension polydispersivity, bearing N-methyl-D-glucamine functionalities.
2. Experimental 2.1. Chemical reagents All reagents were purchased from Sigma-Aldrich Co. Monomers were distilled prior to their use. Other reagents were used as received. Solvents were purchased from POCh Gliwice, Poland, and use without any purification.
194
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2.2. Polymerization and modification reactions 2.2.1. Synthesis of VBC/S/DVB terpolymer VBC/S/DVB matrices were prepared from vinylbenzyl chloride (VBC), styrene (S) and divinylbenzene (DVB) using membrane emulsification followed by suspension polymerization. The obtained polymers had 6 wt.% of crosslinker agent with the following ratio of VBC to S 5:5 (called as Polymer A), 6:4 (Polymer B) and 7:3 (Polymer C). Polymerization was initiated by azoizobutyronitrile (1% wt). More details on the synthesis procedure can be found elsewhere [21]. Membrane emulsification process was carried in a unit supplied by Micropore Ltd, equipped with metal membrane [22] with regularly arranged pores of 5 μm size. The droplets of monomers were polymerized according to the suspension polymerization protocol, at the following temperature regimes: 60 °C–2 h, 85 °C–12 h. Polymerized microspheres were rinsed with hot water, cold water and acetone, dried and extracted with hot toluene in a Soxhlet apparatus. 2.2.2. Preparation of the resins The polymeric resins were prepared by modification of polymer matrices at reflux (BP) and in a microwave reactor (MW). In the first case, the VBC/S/DVB spheres were swollen in dioxane, DIO, and reacted with N-methyl-D-glucamine, NMDG. The reaction was carried out in 1:1 mixture of dioxane:water, with 10 times molar excess of NMDG in respect to chloride content. The reaction was carried out for 2 h. After modification, resin was washed with ethanol, ethanol/water (1:1) and water. Finally, spheres were placed in a column and washed with 1 M HCl and 1 M NaOH aqueous solutions. For modification in a microwave reactor, the spheres were swollen in solution of NMDG in dimethyl sulfoxide, DMSO, or dimethyl formamide, DMF. The same excess of amine to chloride groups as in BP route was set. The modification was carried out in the microwave reactor for 10 min at energy of 100 W. After that time the resins were washed with ethanol, ethanol/water (1:1) and water, and rinsed with 1 M HCl and 1 M NaOH solutions. 2.2.3. Analyses The morphology of the dispersed phase was investigated by SEM microscope. The average diameters and size distribution of microspheres were measured using a Mastersizer X (Melvern Instruments GmbH, Germany). Water regain was measured using centrifugation method, by placing 0.5 g of swollen polymer in a fritted-glass column and centrifuging it for 5 min with 3000 rpm. The sample was weighted, and dried at 105 °C for 24 h. After cooling to room temperature the sample was weighted again. Water regain was calculated as (mw − md)/md, where mw is the weight of swollen polymer after centrifugation, and md is the dry weight of polymer [23]. Chloride content in polymers was measured by burning about 20 mg of dry sample in a flask containing 3% hydrogen peroxide solution. The content of chloride was determined by the Volhardt's method [24]. Nitrogen content in the resins was determinate by the Kjeldahl method after mineralization of the sample of resin (about 200 mg) in concentrated sulfuric acid containing copper sulfate and potassium sulfate [25]. FTIR spectra of resins were recorded on a Perkin Elmer Sys. 2000 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was analyzed on a SPES ESCA system equipped with a PHOIBOS 100 analyzer with X-ray generated by Mg anode at 10 kV and 200 W. The takeoff angle was set as 90o. Pressure in the analysis chamber was kept at 5 10− 9 mbar level and pass energy was set at 30 eV. 2.2.4. Evaluation of sorption properties The sorption capacity of resins towards boron (B) was determined by contacting the samples with 20 mL of B solution (2 mg/L). Boron
solution was prepared with MiliQ water, and re-crystallized boric acid. After 24 h, the samples were filtered and concentration of B was determined. The boron concentration was detected by use of Curcumine method (λmax = 543 nm) [26]. Jasco V-530 spectrophotometer was applied. The range of boron standards was kept between 0.1 and 2.0 mg B/L. Boron sorption per gram of dry resin (q) and distribution coefficient (K) were calculated. 2.2.5. Sorption isotherms For the determination of sorption isotherms, different amounts of samples (from 0.125 to 1.75 g dry resin/L solution) were shaken with 50 mL boon solution (2 mg/L of B) at room temperature for 24 h. After that time the resin was filtered and concentration of boron was determinate. The maximum boron uptake (qmax) was calculated according to Langmuir equation. 2.2.6. Sorption kinetics Sorption kinetics were determined for selected resins. Typical procedure was as follows: a flask with a capacity of 250 mL was filled with resin to give a suspension of 0.5 g dry polymer/L. Boron solution (2 mg/L, 200 mL) was added to the flask and mixed gently. The samples of solution were taken out at different time to determine boron concentration. 3. Results and discussion 3.1. Preparation of chelating resins Preparation of monodisperse microspheres having a relatively small size is one of the important tasks for materials engineering. Using membrane emulsification of monomers mixture and polymerization of emulsion one is able to obtain polymer beads with regular shape (see Figs. 1, 2) and very narrow size distribution. Using this method three types of polymer microspheres were synthesized. They had gel-like structure and contained various amount of VBC as is described in Table 1. A high degree of crosslinking was needed to get the mechanically resistant polymer beads while the lack of neutral solvents guaranteed their solid structure (see the example in Fig. 1). It was noted that the average diameter of microspheres was 30 μm and was about 6 times larger than the diameter of pores in the membrane. The obtained spheres were characterized by their size and polydispersity, water regain and chloride content. These properties are listed in Table 1 also. Analysis of chloride content showed that sphere A have 2.30 mmol/g, sphere B — 3.45 mmol/g and sphere C — 3.60 mmol/g of chloride. These values correspond to 76%, 85% and 86% yield of VBC incorporation to the polymer matrix, respectively. As chloride content in samples B and C was almost the same, it was decided to not increase more vinylbenzyl chloride concentration in the monomer mixture.
Fig. 1. SEM picture of polymer B.
J. Wolska, M. Bryjak / Desalination 283 (2011) 193–197
195
Trans. [%]
a b c
4000
3000
2000
1000
0
Wavelength [cm-1] Fig. 3. FTIR spectra for: unmodified polymer C (a), resin C/BP (b), resin C/DMSO/MW (c). Fig. 2. SEM picture of polymer C.
Water regain for all evaluated matrices was as small as 11%. It confirmed the gel structure of the prepared spheres. Modification of obtained polymers was carried out by reacting NMDG with methyl chloride groups of VBC. Two methods of modification were employed: i) at reflux, and ii) in a microwave reactor. The properties of obtained resins are summarized in Table 2. Depending on the used method, a different degree of substitution of chloromethyl groups was observed. The best method seems to be modification in boiling point. However, for DMSO as solvent the microwave modification offers similar results. The water regain of obtained resins was related to used method and the kind of polymer matrix. The resins with high content of nitrogen were more hydrophilic and had water regain on the range of 0.4–0.59 g/g. It looks that B and C matrices are the best materials for preparation of NMDG substituted resins. The chemical structure of C resin, before and after its modification, was investigated by FTIR method. In Fig. 3a, FTIR spectrum of unmodified matrix C is shown. Polymers having methyl chloride groups gave an absorption band at 1265 cm− 1 that is attributed to C\Cl bond [27,28]. The peak disappeared when \Cl was substituted by NMDG (Fig. 3b and c). When the modification developed the changes were noted in the 2790–700 cm− 1 region. On each spectrum, valence band of \CH was detected at the 2920–2850 cm− 1 region. As a result of chloride substitution by NMDG, this band became stronger. In the case
Table 1 Characteristic of polymers VBC/S/DVB. Polymer
VBC:S ratio (wt.:wt.)
Chloride content [mmol/g]
Average diameter [μm]
SPAN
A B C
5:5 6:4 7:3
2.30 3.45 3.66
28 26 30
0.8 0.9 1.0
of C/BP resin (Fig. 3b), it was no band at 1265 cm− 1. It confirmed the complete replacement of chloride by N-methyl-D-glucamine. Survey XPS scans allowed to estimate the surface atom composition of the studied materials. The results are presented in Table 3. Following the data, one notes relatively high concentration of nitrogen for C/BP and C/DMSO/MW resins. It means that these samples contained the highest contents of N-methyl-D-glucamine functionalities. What is more, the NMDG substitution was completed in these samples as no Cl2p peak was found in the spectra. 3.2. Evaluation of the sorption properties The sorption properties of obtained resins were checked to evaluate their usefulness for the hybrid systems. To determine it, a boric acid stock solution was prepared. It contained 2 mg B/L in H2O and its pH value was adjusted to 8.2. The obtained results, presented as distribution coefficient K = (Co − Ce)/Ce where Co and Ce are boron concentration before and after sorption, are shown in Table 4. It is seen that the resin obtained from polymer C, has the strongest affinity towards boric acid. The highest distribution coefficient (K) was noted for two its types: C/DMSO/MW and C/BP. 3.3. Sorption isotherms and kinetics studies The above presented studies forced us to investigate the sorption process more deeply. Fig. 4 shows sorption isotherms of resins at room temperature. As expected all studied sorbents showed Langmuir-shape relationship. For this reason Langmuir equation was applied to calculate the sorbent capacity value (qmax):
q=
qmax ⋅ c a+c
ð1Þ
where q is the boron uptake (mmol/gresin), c is the equilibrium concentration (mmol/L), and a is a constant (mmol/L). The values of qmax calculated for investigated resins are listed in Table 5.
Table 2 Characteristics of polymeric resins. Resin
Matrix
Solvent
Method of modification
Nitrogen content [mmol/g]
Chloride content [mmol/g]
Water regain [g/g]
A/BP A/DMSO/MW A/DMF/MW B/BP B/DMSO/MW B/DMF/MW C/BP C/DMSO/MW C/DMF/MW
A
DIO DMSO DMF DIO DMSO DMF DIO DMSO DMF
BP MW MW BP MW MW PB MW MW
1.77 0.88 0.32 2.08 1.16 0.47 2.12 2.05 0.45
0 1.21 1.51 0 0.80 1.90 0 0.03 2.70
0.55 0.26 0.18 0.51 0.38 0.21 0.55 0.46 0.19
B
C
Table 3 XPS surface composition in VBC/S/DVB matrix and resins. Sample
B C B/BP B/DMF/MW B/DMSO/MW C/BP C/DMSO/MW
Sample composition [%] C1s
O1s
Cl2p
N1s
75.7 76.8 69.9 69.8 71.5 75.4 78.3
23.8 23.4 28.8 28.8 26.7 22.5 19.0
0.5 0.8 – 0.3 0.1 – –
– – 1.3 1.1 1.7 2.1 2.7
196
J. Wolska, M. Bryjak / Desalination 283 (2011) 193–197 Table 4 Sorption of boron by resins from 2 mg/L boron solutions.
Table 5 Maximum uptake of boron by evaluated resins.
Resin
Distribution coefficient, K
Resin
qmax [mmol/gresin]
A/BP A/DMSO/MW A/DMF/MW B/BP B/DMSO/MW B/DMF/MW C/BP C/DMSO/MW C/DMF/MW
15.6 9.9 1.5 11.7 8.2 0.11 97 145 1.5
A/BP A/DMSO/MW A/DMF/MW B/BP B/DMSO/MW B/DMF/MW C/BP C/DMSO/MW C/DMF/MW
0.25 – – 0.26 0.21 – 1.04 1.06 –
For both sorbents, a slight decrease in sorption properties was observed after the first cycle. The sorption of boron was reduced to 96% for C/DMSO/MW and to 93% for C/BP of basic value and remained constant for the next cycles. It is noteworthy that after the fifth cycle, sorption of boron was still high: the C/DMSO/MW resin was able to adsorb 1.0 mmol B/g while the C/BP resin — 0.93 mmol B/g. These results confirm that NMDG ligands were covalently linked to the polymer matrix and polymer microspheres were resistant to the pH-shocks. 4. Conclusion Process of suspension formation by membrane emulsification followed by suspension polymerization offers particles with 30 μm diameter and narrow size distribution. Subsequent modification of prepared microspheres results in obtaining good sorbents for boron removal and these sorbents can be used in the hybrid systems. The 1,2 1 0,8
C/C0
At the first glance, both C/BP and C/DMSO/MW resins show similar sorption capacity as commercial sorbents. Roughly recalculation of the sorption capacity for Purolite S108, Diaion 2B, IRA-743 or Dowex XUS shows that this value is somewhere between 0.8 and 1.0 mmol/g. The more carefully check of data in Table 5 reveals that resins capacities are in relationship to nitrogen contents, that can be roughly identified as NMDG concentration. However, the direct comparison of qmax with nitrogen concentration (see Table 2) allows to conclude that not everyone ligand participate in sorption. Access to the internal functional groups was not easy due to the solid structure of polymer matrix. For C/DMSO/MW resin, the ratio of qmax to nitrogen content was 0.65 while for C/BP resin it reached smaller value. It means that ca. 50% of NMGD ligands participated in boron binding. The kinetics of boron sorption was the next property that was evaluated. The studies were conducted for the best materials: C/DMSO/MW and C/BP. Fig. 5 shows the percentages of boron uptake as a function of time. Sorption of boron by both studied resins was very fast. It can be seen, that C/DMSO/MW resin reduced boron concentration about twotimes faster than C/BP resin did. It is expected that this phenomenon is related to better accessibility of NMDG ligands for the first resin. Similar results were obtained when distribution coefficients are compared (see Table 4).
3.4. Resins stability in sorption/desorption cycles
0,6
C/M/DMSO
C/BP
0,4
Complete desorption of boron from resins is usually generated by 5% H2SO4 solution. After it, the resin should be equilibrated with solution having pH as high as 8.2. Such procedure exposes ionexchange resins to different pH conditions and may affect their sorption properties. In order to check the resins stability, five sorption/desorption cycles were conducted for both selected sorbents. After each cycle, the sorption capacity was evaluated. To make the comparison easy, sorption was normalized to the fresh resin (Fig. 6).
1 C/DMSO/M
0,8
C/BP B/BP
0,6
B/DMSO/M
0,4
A/BP A/DMSO/M
0,2
0 0
500
1000
1500
TIME [min] Fig. 5. Kinetics of sorption of boron on C/BP and C/DMSO/MW.
procent of initial max. uptake
boron uptake [mmol/g]
1,2
0,2
C/DMSO/M
100
C/BP
90 80 70 60 50 1
0 0
0,025
0,05
0,075
0,1
0,125
0,15
0,175
2
3
4
5
sorption/desorption cycle
equlibrium conc. [mmol/L] Fig. 4. Sorption isotherms of boron on obtained resins.
Fig. 6. Sorption properties of resins: C/DMSO/MW and C/BP in five cycles of sorption and desorption.
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