Synthesis of polymeric microparticles for water purification

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Microporous and Mesoporous Materials 110 (2008) 141–149 www.elsevier.com/locate/micromeso

Synthesis of polymeric microparticles for water purification Olga Kammona a, Elpiniki Dini a, Costas Kiparissides

a,*

, Rosa Allabashi

b

a

b

Department of Chemical Engineering, Aristotle University of Thessaloniki and Chemical Process Engineering Research Institute, P.O. Box 472, 54124 Thessaloniki, Greece The Institute for Water Provision, Department for Water Pollution Control, IWGA-SIG, University of Agricultural Science, Muthgasse 18, A-1190 Vienna, Austria Received 29 May 2007; received in revised form 27 September 2007; accepted 8 October 2007 Available online 18 October 2007

Abstract In the present study, highly crosslinked poly(styrene/meta-diisopropylbenzene) P(St/mDIB) microparticles were prepared employing both an emulsifier-free emulsion polymerization and a single-step swelling polymerization process. The effect of the crosslinker concentration on the particle size and morphology was examined experimentally. Polystyrene microparticles were also prepared by emulsion polymerization in the presence of b-cyclodextrin, P(St/b-CD). The ability of the synthesized polymeric microparticles to adsorb various organic pollutants (e.g., styrene, trihalomethanes and chlorinated volatile organic compounds) from aqueous solutions was examined by gas chromatography (GC). It was shown that the particles exhibit a moderate affinity for styrene, a low affinity for trihalomethanes and a high affinity for volatile organic compounds (VOCs), higher than that of activated carbon (AC). The polymeric microparticles were subsequently impregnated into ceramic, SiC/TiO2 and Al2O3, filters. The hybrid filters were used for the purification of potable water and their adsorption capacity was examined by high performance liquid chromatography (HPLC). It was shown that a SiC/TiO2 filter, impregnated with P(St/mDIB) microparticles, exhibits increased adsorption efficiency as compared to the empty ceramic filter.  2007 Elsevier Inc. All rights reserved. Keywords: Polymeric microparticles; Poly(styrene/meta-diisopropyl benzene); b-cyclodextrin; Hybrid filters; Water purification

1. Introduction Water treatment has been motivated by the occasional presence of undesirable tastes and odors in drinking waters resulting from natural substances, such as those produced by algae and the decay of vegetation. This problem today, has been intensified by increased consumption of water per capita that has led to the use of less desirable water sources in order to provide adequate amounts of supply. Additionally, pollution of available water sources has increased dramatically due to the increased use and evolution of a broader and more complicated spectrum of organic/inorganic pollutants as a consequence of industrialization [1]. Typical organic chemicals that appear in the list of water contaminants are polycyclic aromatic hydrocarbons *

Corresponding author. Tel.: +30 2310 996211; fax: +30 2310 996198. E-mail address: [email protected] (C. Kiparissides).

1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.10.011

(PAHs), acrylamide, styrene, (dichloro)benzene, polychlorinated biphenyls (PCBs), pesticides, chlorinated volatile organic compounds (e.g., tri- and tetrachloroethylene), trihalomethanes, etc. According to the US Environmental Protection Agency, the continuous consumption of water containing an organic contaminant in excess of a few parts per billion (ppb) could lead to serious health problems (e.g., increased risk of cancer, blood problems, liver, kidney and spleen damage, immune deficiencies, reproductive or nervous system difficulties, etc.) depending on the type of the pollutant [2]. According to the above, there is an emerging need for the development of novel purification systems, (e.g., hybrid filters consisting of ceramic carriers impregnated with colloidal micro-, nanogels), which can effectively remove organic contaminants at concentrations as low as a few ppb. Colloidal microgels are crosslinked polymeric particles that can be swollen in the presence of a solvent. Due to

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the swelling behavior of the polymer network, colloidal microgels have a wide range of applications, including controlled drug delivery, removal of various molecules and ions from aqueous solutions, etc. The physicochemical behavior of these materials arises from changes in their size in response to external stimuli such as temperature, pH, ionic strength, moisture, etc. Hydrophobic microgels can be prepared by various polymerization techniques (e.g., suspension polymerization [3], multi- [4–7] or single-step swelling polymerization processes [8,9], emulsion polymerization [10,11], etc.) in the presence of a suitable porogen. The single-step swelling polymerization process [8,9] is suitable for the preparation of highly crosslinked particles of controlled particle morphology, using a variety of monomers and porogens. According to this method, monodisperse polymeric nanoparticles are initially prepared via emulsion polymerization. Oil in water emulsion is then added to the microparticles which become swollen by the oil phase consisting of monomer, crosslinker, porogen and an oil soluble initiator. Finally, the swollen microparticles are polymerized. In emulsion polymerization, an aqueous dispersion of a monomer or a mixture of monomers is converted by freeradical polymerization into a stable dispersion of polymeric particles with diameters in the range of 50 nm–5 lm. A conventional emulsion polymerization recipe comprises four essential ingredients, namely, the dispersion medium, the monomer, the initiator and the emulsifier. The emulsifier concentration employed is usually in excess of the critical micelle concentration (CMC). Polymer latexes prepared in the presence of high surfactant concentrations have typically solid contents as high as 50% and particles in the size range of 50–500 nm. On the other hand, polymer latexes prepared by emulsifier-free emulsion polymerization have sizes in the range of 100 nm–5 lm and are electrostatically stabilized by the charged initiator groups located on the particles’ surface [12,13]. Recently, Rimer and Tattersall [14] described the synthesis of polymeric nanoparticles by emulsion polymerization using b-cyclodextrin as a surfactant. Cyclodextrins (CD) are cyclic oligomers composed of six, seven or eight anhydrous glucopyranosyl units (AGU) (known as a-, b-, c-CD, respectively) linked together by a-1,4-linkages [15]. They can solubilize hydrophobic compounds in aqueous media by complexation within their hydrophobic cavities. The presence of cyclodextrin on the surface of hydrophobic polymeric particles renders them as potential candidates for the removal of organic contaminants from aqueous solutions. In the present study, highly crosslinked poly(styrene/ meta-diisopropylbenzene) P(St/mDIB) microparticles were prepared employing an emulsifier-free emulsion polymerization and a single-step swelling polymerization process. Polystyrene microparticles were also prepared by emulsion polymerization in the presence of b-cyclodextrin, P(St/bCD). Styrene and selected chlorinated volatile organic compounds (e.g., tri- and tetrachloroethylene) and trihalo-

methanes (e.g., chloroform and bromoform) were used as water pollutants in order to examine the adsorption efficiency of the produced polymeric microparticles. The adsorption capacity of the hybrid filters (i.e., ceramic filters impregnated with polymeric microparticles) was also studied. 2. Experimental 2.1. Materials Styrene (Aldrich, +99%) was used as received without any further purification. Potassium persulfate (Fluka) and azobisisobutyronitrile (Riedel) were used as initiators and meta-diisopropyl benzene (Aldrich) was used as crosslinker. Sodium dodecyl sulfate (Aldrich) and polyvinyl alcohol (MW:100,000, Fluka) were used as surfactants, 1pentanol (Aldrich) as co-surfactant and toluene (Aldrich) as porogen. Tri-, tetrachloroethylene, chloroform and bromoform (Aldrich) were used as pollutants and activated carbon (G-60, 100 mesh) in powder form (Aldrich) was used as model adsorbent. In all polymerization experiments, nitrogen of high purity (99.999%) was employed as purging gas. 2.2. Polymerization procedure The polymerization experiments were carried out in laboratory scale, water-jacketed, glass reactors of 0.5 and 1 L (Normschliff Geratebau Wertheim) equipped with a sixblade impeller, an overhead condenser and a nitrogen purge line. The reaction mixture was thermostated to within ±0.05 C with the aid of a constant temperature bath (Julabo F32 HC). 2.2.1. (Emulsifier-free) emulsion polymerization For the synthesis of the poly(styrene/meta-diisopropylbenzene) P(St/mDIB) microparticles, distilled water was introduced to the reactor followed by the addition of a mixture consisting of styrene (St), meta-diisopropylbenzene (mDIB) and toluene. On the other hand, for the synthesis of the poly(styrene/b-cyclodextrin) P(St/bCD) microparticles, a predetermined amount of b-cyclodextrin (b-CD) (5.33% wt based on St) dissolved in distilled water, was introduced to the reactor followed by the addition of styrene. In each case, the reactor was purged with nitrogen, while the emulsion was being heated. As soon as the temperature of the reaction mixture reached a pre-specified value (e.g., 70 C), the nitrogen flow was stopped and the initiator solution (e.g., potassium persulfate, KPS) was injected into the reactor. The polymerization reaction was carried out to high conversions. After the completion of the polymerization, the polymeric particles (e.g., P(St/ mDIB), P(St/bCD)) were left to dry at room temperature. The P(St/bCD) microparticles were then washed with water to remove traces of unbound b-CD whereas, the Soxhlet Extraction technique was applied for the removal

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of toluene from the P(St/mDIB) particles. The process consisted of a continuous extraction with methanol for 10 h. After the purification procedure the polymeric particles were collected in the form of free flowing powder. The effect of the crosslinker concentration on the particle size and morphology was examined experimentally. The polymerization conditions employed for the synthesis of the P(St/mDIB) and P(St/bCD) microparticles by (emulsifier-free) emulsion polymerization are presented in Table 1. 2.2.2. Single-step swelling polymerization process Monodisperse polystyrene (PS) nanoparticles with average diameter approximately 1 lm were initially prepared by emulsifier-free emulsion polymerization, using KPS as initiator, in order to be used as seeds. A small amount of the PS latex was diluted down to 10% wt solids and was introduced to the reactor. The oil phase consisting of St, m-DIB, toluene, 1-pentanol and azobisisobutyronitrile (AIBN) was emulsified for 2 min in an aqueous solution of PVA (0.6% wt) and SDS (0.18% wt) by means of an ultrasonic disrupter. The emulsion thus prepared was added dropwise, under stirring (e.g., 250 rpm), to the PS latex, in three equal time intervals. The reaction mixture was kept under stirring (e.g., 250 rpm) to aid the absorption of the emulsified organic phase by the PS particles and then it was allowed to polymerize by increasing the temperature to 70 C. The particles thus prepared needed successive washing cycles in order to remove the surfactants (e.g., PVA and

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SDS) and the diluent (e.g., toluene). More specifically, the final product was thoroughly washed with hot water and centrifuged. The collected microparticles were then dispersed in acetone, filtered and left to dry. The Soxhlet Extraction technique was then applied for the removal of toluene from the P(St/mDIB) microparticles. The process consisted of a continuous extraction with tetrahydrofuran (THF) for 10 h. After the purification procedure, the P(St/mDIB) particles were left to dry at room temperature. The effect of the crosslinker concentration on the particle morphology was examined experimentally. The polymerization conditions employed for the synthesis of the P(St/mDIB) microparticles by the single-step swelling polymerization process are presented in Table 2. 2.3. Polymer characterization The size distribution of the purified P(St/mDIB) and P(St/b-CD) microparticles was measured by laser diffraction (Malvern Mastersizer 2000). The instrument covered a size range from 20 nm to 2000 lm. Prior to being measured, the dried particles were re-dispersed in distilled water by means of an ultrasonic disrupter. The surface morphology of the polymeric microparticles was assessed by scanning electron microscopy (Jeol JSM6300). Before being examined, the purified particles were coated with gold layers under vacuum. The method of nitrogen adsorption at its boiling point was employed for the determination of the surface area of the P(St/mDIB) and P(St/b-CD) particles. The adsorption

Table 1 Polymerization conditions employed for the synthesis of P(St/mDIB) and P(St/bCD) porous microparticles by (emulsifier-free) emulsion polymerization Experiment

qc (%)a

Volume ratio toluene/ (St + mDIB)

Volume ratio org. phase/Aq. phase

Mass ratio KPS/ (St + mDIB)

Dmean (lm)

Surf. area (m2/ g)

EP-PSmDIB007 EP-PSmDIB008 EP-PSmDIB009 EP-PSmDIB010 EP-PSbCD001

13.16

0.251

0.235

2.08

1.648

6.34

21.94

0.251

0.235

2.08

1.874

6.01

32.91

0.251

0.234

2.08

1.545

5.83

47.02

0.251

0.234

2.08

0.437

5.44

–

–

0.2637

0.90

0.292

9.12

a

% degree of crosslinking = (moles of crosslinker/moles of monomer) * 100.

Table 2 Polymerization conditions employed for the synthesis of P(St/mDIB) porous microparticles by the single-step swelling polymerization process Experiment

qc (%)a

Volume ratio toluene/(St + mDIB)

Volume ratio org. phase/Aq. phase

Mass ratio AIBN/(St + mDIB)

Dmean (lm)

SWP-PSmDIB-001 SWP-PSmDIB-002 SWP-PSmDIB-004 SWP-PSmDIB-005 SWP-PSmDIB-006

16.50 10.16 23.90 32.90 43.90

1.055 1.054 1.058 1.058 1.059

0.0838 0.0839 0.0837 0.0837 0.0837

2 2 2 2 2

3.534 – 2.532 – –

a

% degree of crosslinking = (moles of crosslinker/moles of monomer) * 100.

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method is particularly relevant to powders having particle sizes about 1 lm, where methods based on optical microscopy are inapplicable. The nitrogen adsorption/desorption isotherms were obtained using a quantachrome autosorb automated gas sorption system. The cross sectional area ˚ 2. of the nitrogen molecule was considered equal to 16.2 A Samples of the polymeric microparticles were typically outgassed at 0 C for 18 h. The total analysis time (adsorption/ desorption of particles) was approximately 5 h. The linear form of the BET equation was employed in order to calculate the surface areas. The presence of b-CD on the P(St/b-CD) microparticles was examined by FTIR spectroscopy. Transmission IR absorption spectra were collected at room temperature using a Perkin Elmer FTIR model 2000 spectrophotometer purged with nitrogen and equipped with a wide band MCT detector. KBr pellets were prepared for the P(St/b-CD) microparticles, the PS nanoparticles (i.e., those used as seeds for the single-step swelling polymerization process) and b-CD. Scans were signal averaged at a resolution of 4 cm1 in the range of 4000–450 cm1. The adsorption efficiency of the particles in the selected pollutants (e.g., styrene, chloroform, bromoform, tri- and tetrachloroethylene) was measured employing a GC–FID method (HP 6890 GC, HP 7694 headspace autosampler). Adsorption measurements with activated carbon (AC) in powder form were also performed for comparison purposes. The adsorption experiments were carried out in a batch mode [16]. Pollutant solutions in water at concentrations between 20 and 760 mg/L (depending on the pollutant solubility) were initially prepared and the corner-weighted calibration [17] was adopted to establish the calibration curves. A fixed amount of particles (e.g., 10–50 mg) was then added to pollutant solutions of known concentrations (e.g., 50–500 mg/L) in glass bottles equipped with a screw cap. Nine particle dispersions (i.e., 3 particle quantities · 3 pollutant concentrations) were prepared for each particle type and pollutant examined. The dispersions thus prepared were kept under magnetic stirring at a temperature equal to 25 C for 24 h to reach equilibrium and then were filtered by means of a 0.45 lm PTFE syringe filter. The concentration of the pollutant remaining in the water was measured by GC headspace. The amount of pollutant adsorbed on the polymeric particles was calculated according to the following equation: ðx=mÞ ¼ V ðC 0  C e Þ=m

ð1Þ

where C0 and Ce are the initial and equilibrium concentrations of the pollutant in the solutions (mg/L) respectively, V is the volume of the solution (L), m is the mass of the particles (g) and x/m is the amount of the pollutant adsorbed (mg/g). The equilibrium concentration of the pollutant in the solutions (Ce) was calculated employing the respective calibration curve. The experimental data were then fitted to the linear form of the Freundlich equation (Eq. (2)), which was used in order to plot the Freundlich isotherms:

logðx=mÞ ¼ log K F þ ð1=nÞ logðC e Þ

ð2Þ

where KF is a Freundlich constant which is taken as an indicator of adsorption capacity and 1/n is an empirical constant related to the magnitude of the adsorption driving force which shows the variation of adsorption with concentration. A value of n higher than 1 indicates good adsorption. At this point it should be noted that GC analysis was performed only to EP-PSmDIB-009, SWP-PSmDIB-001 and EP-PSbCD-001 particles. UV spectroscopy (Shimadzu model 2100) was initially employed in order to measure the adsorption capacity of the various particles produced, using styrene as pollutant. These screening measurements indicated that the adsorption efficiency of the aforementioned particles should be more thoroughly examined by GC employing various types of organic pollutants present in potable water (e.g., trihalomethanes, VOCs).

2.4. Impregnation of polymeric particles into ceramic filters The best performing microparticles, regarding their adsorption efficiency, were subsequently impregnated into ceramic carriers employing vacuum filtration, with the aid of a stainless-steel device manufactured by LiqTech (Denmark), thus resulting in the formation of hybrid filters. Two types of ceramic filters were used: (i) Composite filters consisting of silicon carbide (SiC) monoliths (pore sizes: 12 lm) internally coated with a TiO2 membrane (pore sizes: <1.5 lm) (LiqTech, Denmark) and (ii) alumina (Al2O3) filters (pore sizes: 2–3 lm) (CTI S.A., France). Both types of ceramic filters had 52 channels, and their diameters and lengths were equal to 25 mm. A small quantity of particles in the form of powder (e.g., 0.2 g) was dispersed in 20 ml of distilled water. The dispersion was sonicated for at least 1 min prior to being filtered through the ceramic carrier. The impregnated filters were left to dry at room temperature and the excess of particles was removed from their peripheral surface. The morphology of the hybrid filters was then assessed by SEM. Finally, the adsorption efficiency of the hybrid filters was examined by high performance liquid chromatography (HPLC) at the Institute for Water Provision, IWGA-SIG (Vienna, Austria). More specifically, a SiC/TiO2 filter impregnated with the EP-PSmDIB-009 microparticles was placed in the stainless-steel device used for the impregnation. An aqueous tetrachloroethylene solution of initial concentration 100 ppb was then flown through the filter. The flow rate of the solution was controlled with the aid of a peristaltic pump and was set equal to 100 ml/h. 400 ml of tetrachloroethylene solution was filtered in total. The filtration configuration was ‘‘dead-end’’ in order to force the contaminated water stream to flow through the, impregnated with particles, filter wall. At this point, it should be noted that no traces of particles could be observed in the exit stream, indicating that the P(St/mDIB)

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microparticles were not washed away during the filtration process. 3. Results and discussion The effect of the degree of crosslinking on the size distribution of the P(St/mDIB) microparticles prepared by emulsifier-free emulsion polymerization is depicted in Fig. 1. As can be seen, when the nominal value of the degree of crosslinking changes from 13.16% to 32.91% the PSD is not noticeably affected. However, for higher values of qc (e.g., 47.02%) the PSD shifts to smaller sizes due to the lower degree of swelling with monomer and thus lower particle growth rate while, at the same time, becomes broader due to the decreased particle stability caused by the decreased concentration of free polymer chains charged ends. The effect of the degree of crosslinking on the surface morphology of the P(St/mDIB) particles prepared by emulsifier-free emulsion polymerization is depicted in Fig. 2. As

20

Volume (%)

15

ρc: 13.16% ρc: 21.94% ρc: 32.91% ρc: 47.02%

10

5

0 0.01

0.1

1

10

100

Particle Size (μm) Fig. 1. Effect of the degree of crosslinking on the size distribution of the P(St/mDIB) particles prepared by emulsifier-free emulsion polymerization.

145

can be seen, at low degrees of crosslinking, the particle’s surface exhibits a highly porous and irregular character due to the low strength of the polymer network (i.e., it collapses upon the removal of the porogen) (Fig. 2a). On the other hand, at higher values of qc, the particle surface is smoother and maintains its original form upon removal of the porogen due to the presence of a dense 3D polymer network (Fig. 2b). The nitrogen adsorption/desorption isotherms for the P(St/mDIB) microparticles prepared by emulsifier-free emulsion polymerization are shown in Fig. 3. As can be seen, the isotherms are Type II, according to the Brunauer, Deming, Deming and Teller (BDDT) classification, which are characteristic of non-porous solids [18]. The small hysteresis loops observed can be attributed to the presence of superficial cracks and cavities. A SEM photomicrograph of the P(St/bCD) microparticles prepared by emulsion polymerization is shown in the inserted picture of Fig. 4. As can be seen the particles are perfectly spherical with a relatively smooth surface and exhibit a bimodal size distribution. The latter is in agreement with their size distribution as measured by laser diffraction (Fig. 4) and could be attributed to secondary particle nucleation due to the presence of free b-CD molecules in the reaction mixture. The measured nitrogen adsorption/desorption isotherms for the P(St/bCD) microparticles were found to be similar to those of the P(St/mDIB) particles, i.e., Type II exhibiting a small hysteresis loop due to the presence of cracks on the particles’ surface. Fig. 5a shows a SEM photomicrograph of the PS nanoparticles used as seeds for the synthesis of the P(St/mDIB) microparticles by a single-step swelling polymerization process. It is apparent that the PS nanoparticles are perfectly spherical and uniform. The effect of the degree of crosslinking on the surface morphology of the P(St/mDIB) microparticles prepared by a single-step swelling polymerization process was examined. It was shown that, at low degrees of crosslinking, the

Fig. 2. Effect of the degree of crosslinking on the morphology of the P(St/mDIB) particles prepared by emulsifier-free emulsion polymerization: (a) qc: 21.94% and (b) qc: 32.91%.

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O. Kammona et al. / Microporous and Mesoporous Materials 110 (2008) 141–149

12

Adsorbed Nitrogen (mmol/g)

Adsorbed Nitrogen (mmol/g)

14 Adsorption Desorption

12 10 8 6 4 2 0

Adsorption Desorption

10 8 6 4 2 0

0

0.2

0.4

0.6

0.8

1

Relative Pressure, P/Po

0

0.2

0.4

0.6

0.8

1

Relative Pressure, P/Po

Fig. 3. Nitrogen adsorption/desorption isotherms for P(St/mDIB) particles prepared by emulsifier-free emulsion polymerization (a) qc: 13.16% and (b) qc: 47.02%.

12

Volume (%)

10 8 6 4 2 0 0.01

0.1

1

10

100

Particle Size (μm) Fig. 4. Particle size distribution and SEM photomicrograph of the P(St/ bCD) particles prepared by emulsion polymerization.

P(St/mDIB) microparticles are characterized by a highly porous irregular surface morphology (Fig. 5b) which becomes smoother by increasing qc. This again can be explained as follows: At low values of qc, a loose polymer network is formed which collapses upon removal of porogen resulting in a ‘‘nanosponge’’ particle surface morphology (Fig. 5b) whereas, at higher values of qc, the polymer network formed is rather dense and maintains its original form upon drying. Fig. 6 shows the IR spectrum of the P(St/bCD) microparticles in comparison to the spectra of the PS nanoparticles and of b-CD. As can be seen, the absorption bands at 3424 cm1 and in the area of 1200–1000 cm1 appear in the IR spectra of both P(St/bCD) microparticles and bCD. The broad absorption band at 3424 cm1 can be attributed to the stretching vibrations of the OH compounds present in the sugar ring molecules of b-CD. On the other hand, the absorption bands at 1155, 1065 and

Fig. 5. SEM photomicrographs of (a) PS seed nanoparticles and (b) particles prepared by the single-step swelling polymerization process with qc: 10.16%.

Absorbance (A)

O. Kammona et al. / Microporous and Mesoporous Materials 110 (2008) 141–149

1452,49 1493,06

2922,44 3025,85 2849,01 3060,02 3082,41

3423,64

147

756,21 1028,21 1065,57 1155,02

1601,39 1583,18

1943,53

538,89 906,49

1370,86

1803,21 1027,68 1081,08 1158,25

EP-PSbCD-001 3424,05

bCD

2926,42

1656,03

3026,02 3060,15

1367,40 1493,06

1028,48 756,21 1111,25 906,20

1601,58

3082,53 2922,65

539,18 618,11

1452,63

2849,32

PS

4000,0

577,95 697,58

3000

2000

1500

450,0

1000

-1

Wavenumber (cm ) Fig. 6. IR spectra of P(St/b-CD) microparticles, PS nanoparticles and b-CD.

1028 cm1 can be assigned to the stretching of the CO bands of the sugar ring molecules. The existence of these characteristic b-CD absorption bands in the spectrum of the P(St/bCD) microparticles indicates the presence of bCD on the microparticles. In Table 3, the adsorption efficiency of the P(St/mDIB) and P(St/bCD) microparticles is reported in comparison to that of AC. As can be seen, the particles exhibit a moderate affinity for styrene, a low affinity for trihalomethanes (i.e., chloroform and bromoform) and a high affinity for volatile organic compounds (e.g., tri- and tetrachloroethylene), higher than that of AC. At this point it should be noted that the increased adsorption capacity of the P(St/mDIB) microparticles as compared to that of AC is mainly due to the strong hydrophobic forces that develop between the polymeric microparticles and the chlorinated hydrocarbons (solubility parameter d of poly(styrene/divinylbenzene) P(St/DVB) (similar to P(St/mDIB): 9.1 (cal/cm3)1/2 [19] and of trichloroethylene: 9.2 (cal/cm3)1/2 [20]). Regarding the P(St/b-CD) microparticles, the adsorption of the pollutants can be mainly attributed to the surface-bound b-cyclodextrin which is expected to be present on the particle surface in direct contact with the aqueous solution of the pollutant. According to the above, the lower adsorption capacity observed for the P(St/bCD) particles in comparison to that of the P(St/mDIB) microparticles could be ascribed to their lower hydrophobicity. Figs. 7 and 8 depict the Freundlich isotherms of trichloroethylene and bromoform for EP-PSmDIB-009, EPPSbCD-001 and AC respectively. It is apparent that the experimental measurements fit very well to the Freundlich isotherms.

Table 3 Adsorption efficiency results of the P(St/mDIB) and P(St/bCD) microparticles Sample Styrene EP-PSmDIB-009 EP-PSbCD-001 SWP-PSmDIB-001

KF 38.6456 27.7140 58.3579

1/n 0.88694 0.92570 0.57390

Trichloroethylene EP-PSmDIB-009 EP-PSbCD-001 SWP-PSmDIB-001 Activated Carbon

203.376 132.620 156.460 89.39

0.43484 0.44104 0.78340 0.61253

Tetrachloroethylene EP-PSmDIB-009 EP-PSbCD-001 SWP-PSmDIB-001 Activated Carbon

201.604 155.310 62.216 111.276

0.4408 0.9283 0.8284 0.7913

Chloroform EP-PSmDIB-009 EP-PSbCD-001 Activated Carbon Bromoform EP-PSmDIB-009 EP-PSbCD-001 Activated Carbon

3.3083 0.1332 5.9786 20.879 5.225 56.027

1.2812 1.5630 0.9854 0.73895 0.96977 0.61990

Fig. 9 shows a SiC/TiO2 filter impregnated with P(St/ mDIB) microparticles whereas Fig. 10a and b show SEM photomicrographs of a hybrid filter (i.e., Al2O3 filter impregnated with P(St/mDIB) microparticles). As can be seen, the polymeric microparticles are distributed rather homogeneously on the peripheral surface of the ceramic

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3.8 EP-PSmDIB-009 EP-PSβCD-001 Activated Carbon

3.6

Log (x/m)

3.4 3.2 3 2.8 2.6 2.4 2.2 0.4

0.8

1.2

1.6

2

2.4

2.8

Log (Ce) Fig. 7. Freundlich isotherms of trichloroethylene for P(St/mDIB) and P(St/b-CD) particles and activated carbon.

4 EP-PSmDIB-009 EP-PSbCD-001 Activated Carbon

Log (x/m)

3.5

3

2.5

2 1.4

1.6

1.8

2

2.2

2.4

2.6

Log (Ce) Fig. 8. Freundlich isotherms of bromoform for P(St/mDIB) and P(St/bCD) particles and activated carbon.

Fig. 9. SiC/TiO2 filter impregnated with P(St/mDIB) microparticles.

filter and a high percentage of coverage is achieved (Fig. 10a). It should be noted that an increased amount of particles can be found also inside the filter pores (Fig. 10b) as revealed by the examination of the vertical cross section of the impregnated filter. According to the results of Table 3, the P(St/mDIB) microparticles prepared by emulsifier-free emulsion polymerization with a degree of crosslinking equal to 32.91% (EP-PSmDIB-009) perform extremely well regarding the adsorption of tri- and tetrachloroethylene from aqueous solutions. Thus, SiC/TiO2 filters impregnated with these microparticles were sent to IWGA-SIG (Austria) to examine the adsorption efficiency of the polymeric–ceramic hybrid filters by HPLC, using tri- and/or tetrachloroethylene as pollutants. The adsorption results of the hybrid filter (i.e., SiC/TiO2 filter impregnated with EP-PSmDIB-009 microparticles) were compared to those of an empty SiC/ TiO2 filter (Fig. 11). As can be seen from Fig. 11, the hybrid filter exhibits increased adsorption efficiency as compared to the ceramic filter. More specifically, the

Fig. 10. Al2O3 filter containing EP-PSmDIB-009 microparticles: (a) peripheral surface and (b) vertical cross section.

O. Kammona et al. / Microporous and Mesoporous Materials 110 (2008) 141–149

mDIB) microparticles can increase the adsorption efficiency of a SiC/TiO2 filter up to 35% and thus can successfully contribute to the formation of a novel efficient hybrid filter for the removal of VOCs from potable water.

100

80

Adsorption (%)

149

60

Ackowledgment

40

We gratefully acknowledge EU for supporting this research under the Growth Project GDR2-2000-30072. Hybrid filter

20

References

SiC/TiO2 filter

0 0

100

200

300

400

500

Pumped Volume (ml) Fig. 11. Comparison of the adsorption efficiency of a SiC/TiO2 filter impregnated with EP-PSmDIB-009 microparticles with that of an empty SiC/TiO2 filter, using an aqueous tetrachloroethylene solution with initial concentration 100 ppb.

hybrid filter is capable of retaining 70–80% of the pollutant from a tetrachloroethylene solution with concentration 100 ppb whereas the SiC/TiO2 filter retains only 45% of the pollutant under the same experimental conditions. According to the Council Directive 98/83/EC on quality of water intended for human consumption, the concentration of tri- and tetrachloroethylene in drinking water should not exceed 10 ppb [21]. The results of Fig. 11 prove that the hybrid filter (i.e., SiC/TiO2 filter impregnated with P(St/mDIB) microparticles) exhibits increased adsorption efficiency as compared to the empty ceramic filter and thus better meet the demands for efficient removal of tetrachloroethylene from potable water. 4. Conclusions The present study revealed that P(St/bCD) and P(St/ mDIB) microparticles with various degrees of crosslinking can be successfully prepared by (emulsifier-free) emulsion polymerization and a single-step swelling polymerization process. The particles thus prepared were found to have a high affinity for the water contaminants tri- and tetrachloroethylene, higher than that of AC, a moderate affinity for styrene and a low affinity for chloroform and bromoform. In addition, hybrid filters with increased adsorption capacity were successfully prepared via the impregnation of ceramic filters (e.g., SiC/TiO2, Al2O3) with P(St/mDIB) and P(St/bCD) microparticles. It was shown that the P(St/

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