- Email: [email protected]
Octadecyltrimethylammonium surfactant-immobilized cation exchange membranes for solid-phase extraction of phenolic compounds
Microchemical Journal 96 (2010) 290–295
Contents lists available at ScienceDirect
Microchemical Journal 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 / m i c r o c
Octadecyltrimethylammonium surfactant-immobilized cation exchange membranes for solid-phase extraction of phenolic compounds Tien-Yu Wang a, Guan-Liang Chen a, Chao-Chiang Hsu a, Sarah Vied b, Eric D. Conte b, Shing-Yi Suen a,⁎ a b
Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky, KY 42101, USA
a r t i c l e
i n f o
Article history: Received 20 April 2010 Accepted 25 April 2010 Available online 5 May 2010 Keywords: Octadecyltrimethylammonium surfactant Cation exchange membrane Solid-phase extraction Phenolic compound
a b s t r a c t In this study, octadecyltrimethylammonium surfactant was immobilized onto a cation exchange membrane for the application in solid-phase extraction of phenolic compounds. The results indicate that an HCl prewashing step and the use of hydroxide (or methoxide) counter ion could greatly improve the immobilized surfactant capacity. Through elemental and thermogravimetric analyses, the resulted immobilization percentage on the membrane (compared to membrane ion exchange capacity) was about 50, 100, and 150%, respectively, for the feed surfactant amount of 150, 2000, and 5000 μmol (volume = 20 mL). Phenol, 4-nitrophenol, 2,4-dimethylphenol, 2,4-dichlorophenol, 4-chloro-3-methylphenol, and bisphenol A were the tested compounds in a breakthrough volume experiment. The order of the obtained breakthrough volume values is similar to that of Kow values of the phenolic compounds. In the solid-phase extraction process from a feed mixture of 0.1 ppm for 4-nitrophenol, 2,4-dichlorophenol, and 4-chloro-3-methylphenol, high concentration factors and almost complete recoveries were achieved. Moreover, by increasing the membrane volume, a larger sample volume could be processed without any deterioration in performance. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid-phase extraction is currently the most widely adopted method for concentrating the dilute components from a liquid solution [1–8]. Among different interaction modes, hydrophobic interaction is especially popular for concentrating the hydrophobic compounds. For traditional hydrophobic adsorbents such as C18 or C8 silica, polystyrene-divinylbenzene polymers, carbonaceous sorbents, etc., the hydrophobic groups are either inherently existent in the base materials or chemically bonded on the internal surface of adsorbents [2–5,9,10]. In recent years, an alternative for forming hydrophobic adsorbents is to immobilize long carbon-chain ionic surfactants onto ion-exchange resins [6–8]. This preparation process is simple, and mild condition is sufficient for elution. However, conventional bead columns usually have disadvantages such as time-consuming packing, high pressure-driven operation, slow intraparticle diffusion, etc. In this study, membrane was employed as the stationary phase to replace resin beads. Membrane can process liquid samples at higher flow rates (or short residence times) under low pressure drops and avoid nonuniform packing or channeling effects [10–13], making it superior to particle-based systems. Most hydrophobic membranes reported in the literature
⁎ Corresponding author. Tel.: +886 4 22852590; fax: +886 4 22854734. E-mail address: [email protected] (S.-Y. Suen). 0026-265X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2010.04.004
are based on polymeric membranes [14–25]. Their preparation procedures are usually complicated. Based on the above concerns, adopting ion exchange membranes to immobilize ionic surfactants should be a good choice for forming hydrophobic membranes and its related performance is worth an intensive evaluation. This study focused on the immobilization of cationic surfactant onto cation exchange membrane and the parameters influencing the immobilized surfactant capacity. The surfactant used in this work is octadecyltrimethylammonium ion, which may take on an admicellar (bilayer) or hemimicellar (monolayer) arrangement on the internal surface of cation exchange adsorbents. The admicellar/hemimicellar phases have been successfully adopted for extracting components from water such as pesticides, polycyclic aromatic hydrocarbons, phenolic compounds, etc. [26–31]. Hence, the concentration of phenolic compounds from water was conducted to evaluate the efficiency of the hydrophobic membranes prepared in this study. Pollution of phenolic compounds in water usually comes from a wide variety of industrial sources such as pulping, dyeing, plastics, petrochemicals, drugs, herbicides, pesticides, etc. [4,32]. Concentrating phenolic compounds from water may be difficult because of their varying polarities and acidic properties. In this study, the breakthrough volume experiment for selected phenolic compounds was tested to investigate the adsorption properties of the surfactantimmobilized hydrophobic membranes. Furthermore, the enrichment factors and recoveries of phenolic compounds from water in solidphase extraction process were evaluated to explore the possible application of these membranes.
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
291
2. Experimental 2.1. Materials Strong cation exchange P81 membrane, made of cellulose phosphate with 230 μm thickness and H+ counter ions, was purchased from Whatman (Maidstone, Kent, UK). Its measured cation exchange capacity was about 150 μeq per 47 mm disc. The cationic surfactant adopted in this study was octadecyltrimethylammonium bromide (OTAB, C18N+(CH3)3 Br−) from Sigma-Aldrich (St. Louis, MO, USA). Phenol (MW = 94.11), 4-nitrophenol (MW = 139.11), 2,4-dimethylphenol (MW = 122.17), 2,4-dichlorophenol (MW = 163.00), 4-chloro3-methylphenol (MW = 142.59), and bisphenol A (MW = 228.29) were supplied by Sigma-Aldrich and TCI (Tokyo, Japan). Other chemicals and solvents used in this work are of HPLC or analytical grade. 2.2. Octadecyltrimethylammonium surfactant immobilization First, the P81 membrane was rinsed with deionized water for 1 h. Prior to surfactant immobilization, two membrane prewashing steps were tested. In the first case the P81 membrane was immersed in deionized water for 24 h, while in the second case 20 mL of 0.02 M HCl solution was employed. After prewashing, the P81 membrane was rinsed with deionized water again and then dried at room temperature for 24 h. Two octadecyltrimethylammonium surfactants with different counter ions were adopted for surfactant immobilization, one with the bromide counter ion (C18N+(CH3)3 Br−) and the other with the hydroxide or methoxide counter ion (C18N+(CH3)3 OH− or C18N+(CH3)3 OCH− 3 ). Due to the low solubility of octadecyltrimethylammonium surfactant in water, methanol was employed as the solvent. C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 ) was produced by reacting C18N+(CH3)3 Br− and Ag2O at a molar ratio of 2:1 in 20 mL of methanol. The reaction products were the surfactant in the hydroxide and methoxide forms and white AgBr precipitate [31]. The product solution was filtered using a 0.45 μm cellulose acetate filter to remove the precipitate. In the surfactant immobilization experiment, one piece of 47 mm P81 membrane disc (after either H2O or HCl prewashing) was incubated in 20 mL of surfactant solution (in methanol) at a certain feed amount (150, 2000, or 5000 μmol) by shaking at room temperature for 24 h. After immobilization, the membrane was rinsed with deionized water and then dried at room temperature for 24 h.
Fig. 1. Surfactant immobilization results obtained from EA. Feed surfactant amount= 150 μmol, volume= 20 mL.
Malvern Zetasizer Nano System (Worcestershire, UK) equipped with 4 mM He–Ne laser operated at λ = 633 nm. 2.3.4. Thermogravimetric analysis (TGA) In TGA experiment (SSC 5200-H, Seiko, Chiba, Japan), the sample temperature was raised from 30 to 100 °C at a rate of 20 °C/min and kept at 100 °C for 5 min to dry the sample. The temperature was then increased at a rate of 2 °C/min to 500 °C. From the TGA curves, the weight fraction of the immobilized surfactant on membrane could be calculated as S = (Y − X) /(W − X), where Y = the residual weight percentage of the surfactant-immobilized membrane at 500 °C, X = the residual weight percentage of the blank P81 membrane at 500 °C, and W = the residual weight percentage of the pristine surfactant at 500 °C. Thus, the immobilized surfactant amount (mol/disc) = weight of surfactantimmobilized membrane× S ÷ MW of surfactant. Also, the surfactant immobilization percentage (%) based on the membrane IEC = (the immobilized surfactant amount/ the membrane IEC) × 100%. 2.4. Breakthrough volume experiment The equipment for the breakthrough volume experiment included a peristaltic pump (PP-60, Biotop, Taichung, Taiwan), a custom-made polypropylene membrane disc holder (47 mm), a UV/Vis detector (UA-6, ISCO, Lincoln, Nebraska, USA) set at 280 nm, and a computer
2.3. Characterization of octadecyltrimethylammonium surfactant-immobilized membranes 2.3.1. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) To check whether the octadecyltrimethylammonium surfactant was successfully immobilized onto the membrane surface, ATR-FTIR spectra of the prewashed and surfactant-immobilized membranes were measured (Spectrum One, Perkin-Elmer, Wellesley, MA, USA). 2.3.2. Elemental analysis (EA) Analyses on the carbon contents of the prewashed and surfactantimmobilized membranes were conducted via Heraeus CHN-O-S-Rapid Analyzer (Berlin, Germany). The immobilized surfactant amount was determined from the difference in carbon weight between the membranes with and without immobilized surfactants. 2.3.3. Zeta potential The membrane was cut into tiny pieces first. Then, the membrane pieces were immersed in deionized water and vortexed vigorously. The zeta potential of the membrane solution was measured by a
Fig. 2. ATR-FTIR spectra for (a) HCl-prewashed membrane, (b) C18N+(CH3)3 OH− (or C 18 N + (CH 3 ) 3 OCH − 3 )-immobilized membrane, feed amount = 150 μmol, (c) C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount= 2000 μmol, (d) C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount = 5000 μmol. Volume = 20 mL.
292
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
The wavelength was 280 nm. The mobile phase was 50% (v/v) methanol aqueous solution (with 1% formic acid) and the flow rate was 1 mL min− 1. The injection was 100 μl. 3. Results and discussion 3.1. Octadecyltrimethylammonium surfactant immobilization on cation exchange membrane
Fig. 3. Surfactant immobilization results obtained from EA and TGA for the C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membranes under different feed surfactant amounts. Volume = 20 mL.
with chromatographic software. One piece of the surfactant-immobilized membrane (feed surfactant amount = 2000 μmol) was placed in the membrane holder and a silicone O-ring was employed to tightly compress the membrane for preventing lateral fluid leakage. Prior to the experiment, the membrane system was equilibrated by deionized water. In the breakthrough volume experiment, 0.1 ppm of a single phenolic compound in deionized water was introduced into the membrane system at room temperature and 10 mL min− 1, and the breakthrough curve was recorded. The breakthrough volume was defined as the loaded volume at the break point where the effluent phenolic compound concentration reached 10% of its feed value. 2.5. Solid-phase extraction experiment The solid-phase extraction experiment was similar to the breakthrough volume experiment. With 1 piece (or 5 pieces) of surfactant-immobilized membrane (feed surfactant amount = 2000 μmol) in the holder, 100 mL (or 500 mL) of phenolic compound mixture (0.1 ppm for each compound) was loaded at 10 mL min− 1. To recover the phenolic compounds, 7 mL (or 10 mL) of 80% ethanol was passed through the holder at 1 mL min− 1. The effluent in elution stage was collected and analyzed by HPLC. 2.6. HPLC analysis The HPLC system comprised a Waters 600 Controller pump (Milford, MA, USA), a C18 column (Luna C18, 5 μm, 150 × 4.6 mm, Phenomenex, Torrance, CA, USA), a UV–Vis detector (K-2501, Knauer, Berlin, Germany), and a computer with chromatographic software.
3.1.1. Effect of membrane prewashing and effect of surfactant counter ion The commercial cation exchange P81 membrane had a measured ion exchange capacity of ca.150 μeq per 47 mm disc. Two membrane prewashing cases were tested in this study: one by deionized water and the other by 0.02 M HCl, both for 24 h. The long-chain quaternary ammonium surfactant, octadecyltrimethylammonium bromide (C18N+(CH3)3 Br−), was adopted for immobilization onto the P81 membrane. Since C18N+(CH3)3 Br− has low solubility in water, methanol was used as the solvent. Moreover, for evaluating the effect of surfactant counter ion, C18N+(CH3)3 Br− was converted to the hydroxide or methoxide form (C18N+(CH3)3 OH− or C18N+(CH3)3 OCH− 3 ) through a reaction with Ag2O [31]. As shown in Fig. 1, the membrane prewashed by HCl could activate more ion-exchange sites and hence resulted in greater surfactant immobilization capacity. On the other hand, the existence of OH− (or OCH− 3 ) counter ion for surfactant led to a significant improvement on surfactant immobilization. Compared to Br−, the OH− (or OCH− 3 ) counter ions of surfactants exhibited a stronger interaction with the H+ counter ions on the membrane, leaving more active positivelycharged ammonium groups of surfactants to bind with the negatively-charged acid groups on the membrane surface. Based on the above analyses, the HCl prewashing step and the surfactant with OH− (or OCH− 3 ) counter ions were selected as the optimal conditions for C18 surfactant immobilization onto the cation exchange membrane and thus employed in the subsequent experiments. 3.1.2. Effect of feed surfactant amount The ATR-FTIR results are presented in Fig. 2. In comparison with the HCl-prewashed membrane (without immobilized surfactants), the octadecyltrimethylammonium surfactant-immobilized membranes revealed the peaks arising from CH2 vibrations at 2850 and 2920 cm− 1 [33]. This evidence verified the success of surfactant immobilization. Moreover, the CH2 peak intensity increased with the increasing surfactant amount adopted in the immobilization step. The membrane zeta potential investigation also confirmed the successful surfactant immobilization on membrane surface. A zeta potential of −40.8 ± 4.3 mV was obtained for the HCl-prewashed membrane, while those for the membranes with feed surfactant amounts of 150, 2000, and 5000 μmol were −24.4 ± 1.0, 20.6 ± 4.5, and 40.2 ± 4.0 mV, respectively. The membrane zeta potential was changed from negative to positive due to the greater amount of surfactant molecules immobilized.
Fig. 4. Schematic drawing for possible surfactant arrangements at different immobilization percentages.
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
293
To further quantitatively evaluate the effect of feed surfactant amount on the immobilization performance, the surfactant amount immobilized on the membrane was determined by EA and the data are displayed in Fig. 3. The larger the surfactant quantity was loaded, the more surfactant molecules were immobilized onto the cation exchange membrane.
3.1.3. Possible surfactant arrangement on internal membrane surface In Fig. 3, the variation in feed surfactant amount led to a differentdegree surfactant immobilization, e.g. 50, 100, and 150% immobilization results, respectively, for the feed amounts of 150, 2000, and 5000 μmol (volume = 20 mL). That is, the ion-exchange groups on membrane could be covered by the surfactant molecules in a single layer, or even to form the second layer. The membrane zeta potential data reported in the previous section, changing from negative to positive, have supported these results. To more clearly represent these possible interfacial surfactant arrangements under different immobilization percentages, a schematic diagram is depicted in Fig. 4. In 50% immobilization case, the membrane surface would consist of partly the long carbon chains from the immobilized surfactants and partly the unutilized negatively-charged phosphate groups on the cation exchange membrane (hemimicellar form [30,34]). Since a whole surfactant monolayer was not completed, the overall membrane surface charge was still negative. On the other hand, in both 100% and 150% immobilization cases, the surfactant molecules formed a bilayer structure through the hydrophobic interaction among their long carbon chains (hemimicellar/admicellar form [30,34]). Consequently, the overall membrane surface charge became positive. To further investigate the immobilization mechanism, thermogravimetric analysis (TGA) was conducted in this study. The TGA and DTG curves for the surfactant, HCl-prewashed membrane, and surfactant-immobilized membranes are shown in Fig. 5. The HClprewashed P81 (cellulose phosphate) membrane exhibited three weight loss stages: 170–250 °C (about 10% weight loss due to membrane dehydration), 250–320 °C (rate maxima peak at 295 °C, due to the main membrane degradation), and 320–500 °C (for membrane residue decomposition). The above TG phenomenon was similar to that reported in the literature for bacterial cellulosephosphate composite membranes [35]. In contrast, the C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 ) surfactant showed only one sharp weight loss at 200–280 °C (rate maxima peak at 270 °C). In all three cases of surfactant-immobilized membranes, their TGA and DTG curves displayed three weight loss stages: 170–270 °C (rate maxima peak at 230–250 °C), 270–320 °C (rate maxima peak at 300 °C), and 320–500 °C (for membrane residue decomposition). The first DTG peak got bigger with the more surfactant loaded, which should belong to the surfactant decomposition and indicate different immobilized surfactant amounts on the membrane. When a bilayer was formed, there is supposed to be an additional peak in the DTG curve, coming from the interaction between surfactant and membrane [31,36–39]. It may be located in between the two peaks for surfactant decomposition and membrane degradation and could not be observed due to the peak overlapping. Therefore, it is not easy to distinguish the hemimicellar and admicellar surfactant structures from these curves. Furthermore, the quantified surfactant immobilization results from TGA are also plotted in Fig. 3. The values from TGA are close to those obtained from EA. As illustrated in Fig. 4, the surfactantimmobilized membranes with different immobilization percentages exhibited different exposed functional groups. These surfactantimmobilized membranes may be able to serve as mixed-mode (or dynamic) adsorbents with dual interaction modes (ion exchange and hydrophobic) [1,2] for special applications. The ratio of different interaction modes can be simply manipulated by changing the feed surfactant amount.
Fig. 5. TGA and DTG graphs for (a) HCl-prewashed membrane, (b) C18N+(CH3)3 OH− (or + 3 C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount= 150 μmol, (c) C18N (CH3) OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount = 2000 μmol, (d) C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount= 5000 μmol, (e) C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 ) surfactant.
3.2. Breakthrough volume performance The surfactant-immobilized membranes with 100% immobilization were adopted for the adsorption of phenolic compounds in this study. With 100% C18N+(CH3)3 surfactant immobilization, the membrane surface exposed the most hydrophobic tails of surfactants
294
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
Table 1 Breakthrough volume results for phenolic compounds using 1 piece of 47 mm surfactant-immobilized membrane (feed surfactant amount = 2000 μmol). Feed phenolic compound concentration = 0.1 ppm and flow rate = 10 mL min− 1. Phenolic compound
Log Kow
pKa
Breakthrough volumea (mL)
Adsorption %b at the break point
Phenol 4-Nitrophenol 2,4-Dimethylphenol 2,4-Dichlorophenol 4-Chloro-3-methylphenol Bisphenol A
1.50 1.90 2.42 3.08 3.10 3.32
9.99 7.16 10.6 7.85 9.55
13 ± 2 210 ± 5 210 ± 30 230 ± 20 230 ± 30 240 ± 15
70–90 90 80 85 90 80–100
a Breakthrough volume = the loaded volume at the break point where the effluent phenolic compound concentration reached 10% of its feed value. b Adsorption % = the adsorbed amount / the feed amount.
(as shown in Fig. 4), which is suitable as a hydrophobic adsorbent. To evaluate the possible potential for one piece of 47 mm 100% surfactant-immobilized membrane (volume = 0.38 mL) as hydrophobic adsorbent, the breakthrough curve for selected single phenolic compound at 0.1 ppm was measured at 10 mL min− 1 and the corresponding results are listed in Table 1. The log Kow and pKa values for the selected phenolic compounds [1,4,31] are also presented in Table 1. The order of breakthrough volume is similar to that of K ow : phenol b 4-nitrophenol b 2,4-dimethylphenol b 2,4dichlorophenol b 4-chloro-3-methylphenol b bisphenol A. Phenol has the least degree of hydrophobic interaction with the surfactantimmobilized membrane such that its breakthrough volume is very low. For the other five compounds, the breakthrough volumes ranged at 200–250 mL and their adsorption percentages at the break point were above 80%.
3.3. Solid-phase extraction performance Two tests with different membrane volumes, 1 piece of 100% surfactant-immobilized membrane (volume = 0.38 mL) and 5 pieces (volume = 1.9 mL), were conducted in solid-phase extraction process. A water sample of three phenolic compounds (4-nitrophenol, 2,4dichlorophenol, and 4-chloro-3-methylphenol) was loaded at 10 mL min− 1. The feed concentration for each compound was 0.1 ppm (pH ≈ 7). A backpressure of ca. 1.8 bar was created when the above phenolic compound mixture was passed through the empty holder without membranes. For each piece of membrane inserted in the holder, the pressure drop was raised about 0.1 bar. A small volume of 80% ethanol was used to elute and recover the bound phenolic compounds at 1 mL min− 1. The solid-phase extraction results are presented in Table 2 and Fig. 6. For both tests, the recovery of each phenolic compound was close to 100%. In the study of Sojo and Djauhari [10], a 100 mL of acetylated spiked water (1 ppb) was passed through a C18 membrane at 50 mL/min, and finally eluted with 10 mL of acetone at the same
Table 2 Solid-phase extraction results from a mixture of 3 phenolic compounds using 1 or 5 pieces of 47 mm surfactant-immobilized membrane (feed surfactant amount= 2000 μmol). Feed concentration for each phenolic compound= 0.1 ppm, loading flow rate= 10 mL min− 1, elution flow rate = 1 mL min− 1. Surfactantimmobilized membrane
Recovery %c 4-Nitrophenol
2,4-Dichlorophenol
4-Chloro-3methylphenol
1 piece (0.38 mL)a 5 pieces (1.90 mL)b
103 ± 1 104 ± 7
106 ± 5 107 ± 4
99 ± 3 95 ± 5
a b c
Loading volume = 100 mL, elution volume = 7 mL. Loading volume = 500 mL, elution volume = 10 mL. Recovery % = the desorbed amount / the feed amount.
Fig. 6. HPLC analysis results for (a) effluent in elution stage using 5 pieces of 47 mm surfactant-immobilized membranes (feed surfactant amount = 2000 μmol), loading volume = 500 mL, elution volume = 10 mL, (b) effluent in elution stage using 1 piece of 47 mm surfactant-immobilized membrane (feed surfactant amount = 2000 μmol), loading volume = 100 mL, elution volume = 7 mL, (c) the feed mixture of 3 phenolic compounds, feed concentration= 0.1 ppm. 1: solvent, 2: 4-nitrophenol, 3: 4-chloro-3methylphenol, 4: 2,4-dichlorophenol.
flow mode. The recoveries for chlorophenols, chlorocatechols, and chloroguaiacols ranged at 61–102%. Our recovery results are similar to theirs. In our study, the concentration factor was 14 fold (loading volume = 100 mL, and elution volume = 7 mL for a clear elution) using 1 piece of surfactant-immobilized membrane. Increasing the membrane volume 5-fold resulted in a better enrichment effectiveness, about 50 fold (loading volume = 500 mL, and elution volume = 10 mL for a clear elution). This evidence proves that scale up is easy for the membrane-type adsorbent and the performance is not deteriorated.
4. Conclusions Strong cation exchange membranes immobilized with octadecyltrimethylammonium hydroxide (or methoxide) surfactants have been successfully prepared and applied to the solid-phase extraction of phenolic compounds from water. The optimal surfactant immobilization procedure requires an HCl prewashing step (for activating the ion exchange groups) and the use of high surfactant feed amount. High enrichment factors and high recoveries were attained by simply using aqueous alcohol solution as the eluant. The whole process time is short and the pressure drop is low, making this method both timesaving and energy-saving. Moreover, scale up can be easily achieved by stacking more membranes together, without any deterioration in the performance. Consequently, this hemimicellar/admicellar surfactant-immobilized membrane design may provide a beneficial alternative for preconcentrating hydrophobic components such as phenolic compounds. It should be noted that other cation exchange membrane materials could also be adopted for the same application.
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
Acknowledgements The authors are grateful for the financial supports from the National Science Council of Taiwan (Grant No. NSC 97-2221-E-005-041) for Dr. S.-Y. Suen and from the National Science Foundation of the United States (Grant No. OISE-0553369) for Dr. E. D. Conte. Moreover, this work is supported in part by the Ministry of Education, Taiwan under the ATU plan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
N. Li, H.K. Lee, Anal. Chem. 69 (1997) 5193–5199. M.-C. Hennion, J. Chromatogr. A 856 (1999) 3–54. H. Sabik, R. Jeannot, B. Rondeau, J. Chromatogr. A 885 (2000) 217–236. I. Rodriguez, M.P. Llompart, R. Cela, J. Chromatogr. A 885 (2000) 291–304. Y. Cai, G. Jiang, J. Liu, Q. Zhou, Anal. Chem. 75 (2003) 2517–2521. J.K. Autry, E.G. Vaught, E.D. Conte, Microchem. J. 80 (2005) 25–29. Q. Zhao, S.A. Huss, J.K. Autry, K.R. Reid, E.D. Conte, Microchem. J. 80 (2005) 45–54. Q. Zhao, R.T. Philpott, T.D. Oakes, E.D. Conte, Analyst 130 (2005) 750–754. J.J. Kirkland, J.L. Glajch, R.D. Farlee, Anal. Chem. 61 (1989) 2–11. L.E. Sojo, J. Djauhari, J. Chromatogr. A 840 (1999) 21–30. W.P.N. Fernando, M.L. Larrivee, C.F. Poole, Anal. Chem. 65 (1993) 588–595. H. Zou, Q. Luo, D. Zhou, J. Biochem. Biophys. Meth. 49 (2001) 199–240. R. Ghosh, J. Chromatogr. A 952 (2002) 13–27. A. Podgornik, M. Barut, J. Jancar, A. Strancar, T. Tennikova, Anal. Chem. 71 (1999) 2986–2991. [15] T.B. Tennikova, M. Bleha, F. Svec, T.V. Almazova, B.G. Belenkii, J. Chromatogr. 555 (1991) 97–107.
295
[16] M. Kim, K. Saito, S. Furusaki, T. Sato, T. Sugo, I. Ishigaki, J. Chromatogr. 585 (1991) 45–51. [17] T.B. Tennikova, F. Svec, J. Chromatogr. 646 (1993) 279–288. [18] J. Luksa, V. Menart, S. Milicic, B. Kus, V. Gaberc-Porekar, D. Josic, J. Chromatogr. A 661 (1994) 161–168. [19] Q.C. Wang, F. Svec, J.M.J. Frechet, J. Chromatogr. A 669 (1994) 230–235. [20] N. Kubota, M. Kounosu, K. Saito, K. Sugita, K. Watanabe, T. Sugo, J. Chromatogr. A 718 (1995) 27–34. [21] N. Kubota, M. Kounosu, K. Saito, K. Sugita, K. Watanabe, T. Sugo, React. Funct. Polym. 29 (1996) 115–122. [22] N. Kubota, M. Kounosu, K. Saito, K. Sugita, K. Watanabe, T. Sugo, J. Membr. Sci. 134 (1997) 67–73. [23] R. Ghosh, J. Chromatogr. A 923 (2001) 59–64. [24] M.Y. Arica, G.A. Oktem, A. Denizli, Colloids Surf. B 21 (2001) 273–283. [25] G. Bayramoglu, A. Denizli, M.Y. Arica, Polym. Int. 51 (2002) 966–972. [26] J. Danzer, P. Grathwohl, Phys. Chem. Earth 23 (1998) 237–243. [27] T. Saitoh, S. Matsushima, M. Hiraide, J. Chromatogr. A 1040 (2004) 185–191. [28] T. Saitoh, S. Matsushima, M. Hiraide, J. Chromatogr. A 1069 (2005) 271–274. [29] Q. Zhao, J. Simmons, E.D. Conte, J. Chromatogr. A 1132 (2006) 1–7. [30] A. Moral, M.D. Sicilia, S. Rubio, D. Perez-Bendito, Anal. Chim. Acta 608 (2008) 61–72. [31] G.-L. Chen, S.-Y. Suen, S. Vied, K. Pickering, C. Perrin, E.D. Conte, Analyst 134 (2009) 331–336. [32] D. Puig, D. Barceló, J. Chromatogr. A 733 (1996) 371–381. [33] M.A. Osman, G. Seyfang, U.W. Suter, J. Phys. Chem. B 104 (2000) 4433–4439. [34] A. Fan, P. Somasundaran, N.J. Turro, Langmuir 13 (1997) 506–510. [35] H.S. Barud, C.A. Ribeiro, M.S. Crespi, M.A.U. Martines, J. Dexpert-Ghys, R.F.C. Marques, Y. Messaddeq, S.J.L. Ribeiro, J. Therm. Anal. Cal. 87 (2007) 815–818. [36] L. Shen, P.E. Laibinis, T.A. Hatton, Langmuir 15 (1999) 447–453. [37] B. Nikoobakht, M.A. El-Sayed, Langmuir 17 (2001) 6368–6374. [38] M.A. Osman, U.W. Suter, Chem. Mater. 14 (2002) 4408–4415. [39] S.-H. Wu, D.-H. Chen, Chem. Lett. 33 (2004) 406–407.
Contents lists available at ScienceDirect
Microchemical Journal 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 / m i c r o c
Octadecyltrimethylammonium surfactant-immobilized cation exchange membranes for solid-phase extraction of phenolic compounds Tien-Yu Wang a, Guan-Liang Chen a, Chao-Chiang Hsu a, Sarah Vied b, Eric D. Conte b, Shing-Yi Suen a,⁎ a b
Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky, KY 42101, USA
a r t i c l e
i n f o
Article history: Received 20 April 2010 Accepted 25 April 2010 Available online 5 May 2010 Keywords: Octadecyltrimethylammonium surfactant Cation exchange membrane Solid-phase extraction Phenolic compound
a b s t r a c t In this study, octadecyltrimethylammonium surfactant was immobilized onto a cation exchange membrane for the application in solid-phase extraction of phenolic compounds. The results indicate that an HCl prewashing step and the use of hydroxide (or methoxide) counter ion could greatly improve the immobilized surfactant capacity. Through elemental and thermogravimetric analyses, the resulted immobilization percentage on the membrane (compared to membrane ion exchange capacity) was about 50, 100, and 150%, respectively, for the feed surfactant amount of 150, 2000, and 5000 μmol (volume = 20 mL). Phenol, 4-nitrophenol, 2,4-dimethylphenol, 2,4-dichlorophenol, 4-chloro-3-methylphenol, and bisphenol A were the tested compounds in a breakthrough volume experiment. The order of the obtained breakthrough volume values is similar to that of Kow values of the phenolic compounds. In the solid-phase extraction process from a feed mixture of 0.1 ppm for 4-nitrophenol, 2,4-dichlorophenol, and 4-chloro-3-methylphenol, high concentration factors and almost complete recoveries were achieved. Moreover, by increasing the membrane volume, a larger sample volume could be processed without any deterioration in performance. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid-phase extraction is currently the most widely adopted method for concentrating the dilute components from a liquid solution [1–8]. Among different interaction modes, hydrophobic interaction is especially popular for concentrating the hydrophobic compounds. For traditional hydrophobic adsorbents such as C18 or C8 silica, polystyrene-divinylbenzene polymers, carbonaceous sorbents, etc., the hydrophobic groups are either inherently existent in the base materials or chemically bonded on the internal surface of adsorbents [2–5,9,10]. In recent years, an alternative for forming hydrophobic adsorbents is to immobilize long carbon-chain ionic surfactants onto ion-exchange resins [6–8]. This preparation process is simple, and mild condition is sufficient for elution. However, conventional bead columns usually have disadvantages such as time-consuming packing, high pressure-driven operation, slow intraparticle diffusion, etc. In this study, membrane was employed as the stationary phase to replace resin beads. Membrane can process liquid samples at higher flow rates (or short residence times) under low pressure drops and avoid nonuniform packing or channeling effects [10–13], making it superior to particle-based systems. Most hydrophobic membranes reported in the literature
⁎ Corresponding author. Tel.: +886 4 22852590; fax: +886 4 22854734. E-mail address: [email protected] (S.-Y. Suen). 0026-265X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2010.04.004
are based on polymeric membranes [14–25]. Their preparation procedures are usually complicated. Based on the above concerns, adopting ion exchange membranes to immobilize ionic surfactants should be a good choice for forming hydrophobic membranes and its related performance is worth an intensive evaluation. This study focused on the immobilization of cationic surfactant onto cation exchange membrane and the parameters influencing the immobilized surfactant capacity. The surfactant used in this work is octadecyltrimethylammonium ion, which may take on an admicellar (bilayer) or hemimicellar (monolayer) arrangement on the internal surface of cation exchange adsorbents. The admicellar/hemimicellar phases have been successfully adopted for extracting components from water such as pesticides, polycyclic aromatic hydrocarbons, phenolic compounds, etc. [26–31]. Hence, the concentration of phenolic compounds from water was conducted to evaluate the efficiency of the hydrophobic membranes prepared in this study. Pollution of phenolic compounds in water usually comes from a wide variety of industrial sources such as pulping, dyeing, plastics, petrochemicals, drugs, herbicides, pesticides, etc. [4,32]. Concentrating phenolic compounds from water may be difficult because of their varying polarities and acidic properties. In this study, the breakthrough volume experiment for selected phenolic compounds was tested to investigate the adsorption properties of the surfactantimmobilized hydrophobic membranes. Furthermore, the enrichment factors and recoveries of phenolic compounds from water in solidphase extraction process were evaluated to explore the possible application of these membranes.
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
291
2. Experimental 2.1. Materials Strong cation exchange P81 membrane, made of cellulose phosphate with 230 μm thickness and H+ counter ions, was purchased from Whatman (Maidstone, Kent, UK). Its measured cation exchange capacity was about 150 μeq per 47 mm disc. The cationic surfactant adopted in this study was octadecyltrimethylammonium bromide (OTAB, C18N+(CH3)3 Br−) from Sigma-Aldrich (St. Louis, MO, USA). Phenol (MW = 94.11), 4-nitrophenol (MW = 139.11), 2,4-dimethylphenol (MW = 122.17), 2,4-dichlorophenol (MW = 163.00), 4-chloro3-methylphenol (MW = 142.59), and bisphenol A (MW = 228.29) were supplied by Sigma-Aldrich and TCI (Tokyo, Japan). Other chemicals and solvents used in this work are of HPLC or analytical grade. 2.2. Octadecyltrimethylammonium surfactant immobilization First, the P81 membrane was rinsed with deionized water for 1 h. Prior to surfactant immobilization, two membrane prewashing steps were tested. In the first case the P81 membrane was immersed in deionized water for 24 h, while in the second case 20 mL of 0.02 M HCl solution was employed. After prewashing, the P81 membrane was rinsed with deionized water again and then dried at room temperature for 24 h. Two octadecyltrimethylammonium surfactants with different counter ions were adopted for surfactant immobilization, one with the bromide counter ion (C18N+(CH3)3 Br−) and the other with the hydroxide or methoxide counter ion (C18N+(CH3)3 OH− or C18N+(CH3)3 OCH− 3 ). Due to the low solubility of octadecyltrimethylammonium surfactant in water, methanol was employed as the solvent. C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 ) was produced by reacting C18N+(CH3)3 Br− and Ag2O at a molar ratio of 2:1 in 20 mL of methanol. The reaction products were the surfactant in the hydroxide and methoxide forms and white AgBr precipitate [31]. The product solution was filtered using a 0.45 μm cellulose acetate filter to remove the precipitate. In the surfactant immobilization experiment, one piece of 47 mm P81 membrane disc (after either H2O or HCl prewashing) was incubated in 20 mL of surfactant solution (in methanol) at a certain feed amount (150, 2000, or 5000 μmol) by shaking at room temperature for 24 h. After immobilization, the membrane was rinsed with deionized water and then dried at room temperature for 24 h.
Fig. 1. Surfactant immobilization results obtained from EA. Feed surfactant amount= 150 μmol, volume= 20 mL.
Malvern Zetasizer Nano System (Worcestershire, UK) equipped with 4 mM He–Ne laser operated at λ = 633 nm. 2.3.4. Thermogravimetric analysis (TGA) In TGA experiment (SSC 5200-H, Seiko, Chiba, Japan), the sample temperature was raised from 30 to 100 °C at a rate of 20 °C/min and kept at 100 °C for 5 min to dry the sample. The temperature was then increased at a rate of 2 °C/min to 500 °C. From the TGA curves, the weight fraction of the immobilized surfactant on membrane could be calculated as S = (Y − X) /(W − X), where Y = the residual weight percentage of the surfactant-immobilized membrane at 500 °C, X = the residual weight percentage of the blank P81 membrane at 500 °C, and W = the residual weight percentage of the pristine surfactant at 500 °C. Thus, the immobilized surfactant amount (mol/disc) = weight of surfactantimmobilized membrane× S ÷ MW of surfactant. Also, the surfactant immobilization percentage (%) based on the membrane IEC = (the immobilized surfactant amount/ the membrane IEC) × 100%. 2.4. Breakthrough volume experiment The equipment for the breakthrough volume experiment included a peristaltic pump (PP-60, Biotop, Taichung, Taiwan), a custom-made polypropylene membrane disc holder (47 mm), a UV/Vis detector (UA-6, ISCO, Lincoln, Nebraska, USA) set at 280 nm, and a computer
2.3. Characterization of octadecyltrimethylammonium surfactant-immobilized membranes 2.3.1. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) To check whether the octadecyltrimethylammonium surfactant was successfully immobilized onto the membrane surface, ATR-FTIR spectra of the prewashed and surfactant-immobilized membranes were measured (Spectrum One, Perkin-Elmer, Wellesley, MA, USA). 2.3.2. Elemental analysis (EA) Analyses on the carbon contents of the prewashed and surfactantimmobilized membranes were conducted via Heraeus CHN-O-S-Rapid Analyzer (Berlin, Germany). The immobilized surfactant amount was determined from the difference in carbon weight between the membranes with and without immobilized surfactants. 2.3.3. Zeta potential The membrane was cut into tiny pieces first. Then, the membrane pieces were immersed in deionized water and vortexed vigorously. The zeta potential of the membrane solution was measured by a
Fig. 2. ATR-FTIR spectra for (a) HCl-prewashed membrane, (b) C18N+(CH3)3 OH− (or C 18 N + (CH 3 ) 3 OCH − 3 )-immobilized membrane, feed amount = 150 μmol, (c) C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount= 2000 μmol, (d) C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount = 5000 μmol. Volume = 20 mL.
292
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
The wavelength was 280 nm. The mobile phase was 50% (v/v) methanol aqueous solution (with 1% formic acid) and the flow rate was 1 mL min− 1. The injection was 100 μl. 3. Results and discussion 3.1. Octadecyltrimethylammonium surfactant immobilization on cation exchange membrane
Fig. 3. Surfactant immobilization results obtained from EA and TGA for the C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membranes under different feed surfactant amounts. Volume = 20 mL.
with chromatographic software. One piece of the surfactant-immobilized membrane (feed surfactant amount = 2000 μmol) was placed in the membrane holder and a silicone O-ring was employed to tightly compress the membrane for preventing lateral fluid leakage. Prior to the experiment, the membrane system was equilibrated by deionized water. In the breakthrough volume experiment, 0.1 ppm of a single phenolic compound in deionized water was introduced into the membrane system at room temperature and 10 mL min− 1, and the breakthrough curve was recorded. The breakthrough volume was defined as the loaded volume at the break point where the effluent phenolic compound concentration reached 10% of its feed value. 2.5. Solid-phase extraction experiment The solid-phase extraction experiment was similar to the breakthrough volume experiment. With 1 piece (or 5 pieces) of surfactant-immobilized membrane (feed surfactant amount = 2000 μmol) in the holder, 100 mL (or 500 mL) of phenolic compound mixture (0.1 ppm for each compound) was loaded at 10 mL min− 1. To recover the phenolic compounds, 7 mL (or 10 mL) of 80% ethanol was passed through the holder at 1 mL min− 1. The effluent in elution stage was collected and analyzed by HPLC. 2.6. HPLC analysis The HPLC system comprised a Waters 600 Controller pump (Milford, MA, USA), a C18 column (Luna C18, 5 μm, 150 × 4.6 mm, Phenomenex, Torrance, CA, USA), a UV–Vis detector (K-2501, Knauer, Berlin, Germany), and a computer with chromatographic software.
3.1.1. Effect of membrane prewashing and effect of surfactant counter ion The commercial cation exchange P81 membrane had a measured ion exchange capacity of ca.150 μeq per 47 mm disc. Two membrane prewashing cases were tested in this study: one by deionized water and the other by 0.02 M HCl, both for 24 h. The long-chain quaternary ammonium surfactant, octadecyltrimethylammonium bromide (C18N+(CH3)3 Br−), was adopted for immobilization onto the P81 membrane. Since C18N+(CH3)3 Br− has low solubility in water, methanol was used as the solvent. Moreover, for evaluating the effect of surfactant counter ion, C18N+(CH3)3 Br− was converted to the hydroxide or methoxide form (C18N+(CH3)3 OH− or C18N+(CH3)3 OCH− 3 ) through a reaction with Ag2O [31]. As shown in Fig. 1, the membrane prewashed by HCl could activate more ion-exchange sites and hence resulted in greater surfactant immobilization capacity. On the other hand, the existence of OH− (or OCH− 3 ) counter ion for surfactant led to a significant improvement on surfactant immobilization. Compared to Br−, the OH− (or OCH− 3 ) counter ions of surfactants exhibited a stronger interaction with the H+ counter ions on the membrane, leaving more active positivelycharged ammonium groups of surfactants to bind with the negatively-charged acid groups on the membrane surface. Based on the above analyses, the HCl prewashing step and the surfactant with OH− (or OCH− 3 ) counter ions were selected as the optimal conditions for C18 surfactant immobilization onto the cation exchange membrane and thus employed in the subsequent experiments. 3.1.2. Effect of feed surfactant amount The ATR-FTIR results are presented in Fig. 2. In comparison with the HCl-prewashed membrane (without immobilized surfactants), the octadecyltrimethylammonium surfactant-immobilized membranes revealed the peaks arising from CH2 vibrations at 2850 and 2920 cm− 1 [33]. This evidence verified the success of surfactant immobilization. Moreover, the CH2 peak intensity increased with the increasing surfactant amount adopted in the immobilization step. The membrane zeta potential investigation also confirmed the successful surfactant immobilization on membrane surface. A zeta potential of −40.8 ± 4.3 mV was obtained for the HCl-prewashed membrane, while those for the membranes with feed surfactant amounts of 150, 2000, and 5000 μmol were −24.4 ± 1.0, 20.6 ± 4.5, and 40.2 ± 4.0 mV, respectively. The membrane zeta potential was changed from negative to positive due to the greater amount of surfactant molecules immobilized.
Fig. 4. Schematic drawing for possible surfactant arrangements at different immobilization percentages.
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
293
To further quantitatively evaluate the effect of feed surfactant amount on the immobilization performance, the surfactant amount immobilized on the membrane was determined by EA and the data are displayed in Fig. 3. The larger the surfactant quantity was loaded, the more surfactant molecules were immobilized onto the cation exchange membrane.
3.1.3. Possible surfactant arrangement on internal membrane surface In Fig. 3, the variation in feed surfactant amount led to a differentdegree surfactant immobilization, e.g. 50, 100, and 150% immobilization results, respectively, for the feed amounts of 150, 2000, and 5000 μmol (volume = 20 mL). That is, the ion-exchange groups on membrane could be covered by the surfactant molecules in a single layer, or even to form the second layer. The membrane zeta potential data reported in the previous section, changing from negative to positive, have supported these results. To more clearly represent these possible interfacial surfactant arrangements under different immobilization percentages, a schematic diagram is depicted in Fig. 4. In 50% immobilization case, the membrane surface would consist of partly the long carbon chains from the immobilized surfactants and partly the unutilized negatively-charged phosphate groups on the cation exchange membrane (hemimicellar form [30,34]). Since a whole surfactant monolayer was not completed, the overall membrane surface charge was still negative. On the other hand, in both 100% and 150% immobilization cases, the surfactant molecules formed a bilayer structure through the hydrophobic interaction among their long carbon chains (hemimicellar/admicellar form [30,34]). Consequently, the overall membrane surface charge became positive. To further investigate the immobilization mechanism, thermogravimetric analysis (TGA) was conducted in this study. The TGA and DTG curves for the surfactant, HCl-prewashed membrane, and surfactant-immobilized membranes are shown in Fig. 5. The HClprewashed P81 (cellulose phosphate) membrane exhibited three weight loss stages: 170–250 °C (about 10% weight loss due to membrane dehydration), 250–320 °C (rate maxima peak at 295 °C, due to the main membrane degradation), and 320–500 °C (for membrane residue decomposition). The above TG phenomenon was similar to that reported in the literature for bacterial cellulosephosphate composite membranes [35]. In contrast, the C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 ) surfactant showed only one sharp weight loss at 200–280 °C (rate maxima peak at 270 °C). In all three cases of surfactant-immobilized membranes, their TGA and DTG curves displayed three weight loss stages: 170–270 °C (rate maxima peak at 230–250 °C), 270–320 °C (rate maxima peak at 300 °C), and 320–500 °C (for membrane residue decomposition). The first DTG peak got bigger with the more surfactant loaded, which should belong to the surfactant decomposition and indicate different immobilized surfactant amounts on the membrane. When a bilayer was formed, there is supposed to be an additional peak in the DTG curve, coming from the interaction between surfactant and membrane [31,36–39]. It may be located in between the two peaks for surfactant decomposition and membrane degradation and could not be observed due to the peak overlapping. Therefore, it is not easy to distinguish the hemimicellar and admicellar surfactant structures from these curves. Furthermore, the quantified surfactant immobilization results from TGA are also plotted in Fig. 3. The values from TGA are close to those obtained from EA. As illustrated in Fig. 4, the surfactantimmobilized membranes with different immobilization percentages exhibited different exposed functional groups. These surfactantimmobilized membranes may be able to serve as mixed-mode (or dynamic) adsorbents with dual interaction modes (ion exchange and hydrophobic) [1,2] for special applications. The ratio of different interaction modes can be simply manipulated by changing the feed surfactant amount.
Fig. 5. TGA and DTG graphs for (a) HCl-prewashed membrane, (b) C18N+(CH3)3 OH− (or + 3 C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount= 150 μmol, (c) C18N (CH3) OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount = 2000 μmol, (d) C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 )-immobilized membrane, feed amount= 5000 μmol, (e) C18N+(CH3)3 OH− (or C18N+(CH3)3 OCH− 3 ) surfactant.
3.2. Breakthrough volume performance The surfactant-immobilized membranes with 100% immobilization were adopted for the adsorption of phenolic compounds in this study. With 100% C18N+(CH3)3 surfactant immobilization, the membrane surface exposed the most hydrophobic tails of surfactants
294
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
Table 1 Breakthrough volume results for phenolic compounds using 1 piece of 47 mm surfactant-immobilized membrane (feed surfactant amount = 2000 μmol). Feed phenolic compound concentration = 0.1 ppm and flow rate = 10 mL min− 1. Phenolic compound
Log Kow
pKa
Breakthrough volumea (mL)
Adsorption %b at the break point
Phenol 4-Nitrophenol 2,4-Dimethylphenol 2,4-Dichlorophenol 4-Chloro-3-methylphenol Bisphenol A
1.50 1.90 2.42 3.08 3.10 3.32
9.99 7.16 10.6 7.85 9.55
13 ± 2 210 ± 5 210 ± 30 230 ± 20 230 ± 30 240 ± 15
70–90 90 80 85 90 80–100
a Breakthrough volume = the loaded volume at the break point where the effluent phenolic compound concentration reached 10% of its feed value. b Adsorption % = the adsorbed amount / the feed amount.
(as shown in Fig. 4), which is suitable as a hydrophobic adsorbent. To evaluate the possible potential for one piece of 47 mm 100% surfactant-immobilized membrane (volume = 0.38 mL) as hydrophobic adsorbent, the breakthrough curve for selected single phenolic compound at 0.1 ppm was measured at 10 mL min− 1 and the corresponding results are listed in Table 1. The log Kow and pKa values for the selected phenolic compounds [1,4,31] are also presented in Table 1. The order of breakthrough volume is similar to that of K ow : phenol b 4-nitrophenol b 2,4-dimethylphenol b 2,4dichlorophenol b 4-chloro-3-methylphenol b bisphenol A. Phenol has the least degree of hydrophobic interaction with the surfactantimmobilized membrane such that its breakthrough volume is very low. For the other five compounds, the breakthrough volumes ranged at 200–250 mL and their adsorption percentages at the break point were above 80%.
3.3. Solid-phase extraction performance Two tests with different membrane volumes, 1 piece of 100% surfactant-immobilized membrane (volume = 0.38 mL) and 5 pieces (volume = 1.9 mL), were conducted in solid-phase extraction process. A water sample of three phenolic compounds (4-nitrophenol, 2,4dichlorophenol, and 4-chloro-3-methylphenol) was loaded at 10 mL min− 1. The feed concentration for each compound was 0.1 ppm (pH ≈ 7). A backpressure of ca. 1.8 bar was created when the above phenolic compound mixture was passed through the empty holder without membranes. For each piece of membrane inserted in the holder, the pressure drop was raised about 0.1 bar. A small volume of 80% ethanol was used to elute and recover the bound phenolic compounds at 1 mL min− 1. The solid-phase extraction results are presented in Table 2 and Fig. 6. For both tests, the recovery of each phenolic compound was close to 100%. In the study of Sojo and Djauhari [10], a 100 mL of acetylated spiked water (1 ppb) was passed through a C18 membrane at 50 mL/min, and finally eluted with 10 mL of acetone at the same
Table 2 Solid-phase extraction results from a mixture of 3 phenolic compounds using 1 or 5 pieces of 47 mm surfactant-immobilized membrane (feed surfactant amount= 2000 μmol). Feed concentration for each phenolic compound= 0.1 ppm, loading flow rate= 10 mL min− 1, elution flow rate = 1 mL min− 1. Surfactantimmobilized membrane
Recovery %c 4-Nitrophenol
2,4-Dichlorophenol
4-Chloro-3methylphenol
1 piece (0.38 mL)a 5 pieces (1.90 mL)b
103 ± 1 104 ± 7
106 ± 5 107 ± 4
99 ± 3 95 ± 5
a b c
Loading volume = 100 mL, elution volume = 7 mL. Loading volume = 500 mL, elution volume = 10 mL. Recovery % = the desorbed amount / the feed amount.
Fig. 6. HPLC analysis results for (a) effluent in elution stage using 5 pieces of 47 mm surfactant-immobilized membranes (feed surfactant amount = 2000 μmol), loading volume = 500 mL, elution volume = 10 mL, (b) effluent in elution stage using 1 piece of 47 mm surfactant-immobilized membrane (feed surfactant amount = 2000 μmol), loading volume = 100 mL, elution volume = 7 mL, (c) the feed mixture of 3 phenolic compounds, feed concentration= 0.1 ppm. 1: solvent, 2: 4-nitrophenol, 3: 4-chloro-3methylphenol, 4: 2,4-dichlorophenol.
flow mode. The recoveries for chlorophenols, chlorocatechols, and chloroguaiacols ranged at 61–102%. Our recovery results are similar to theirs. In our study, the concentration factor was 14 fold (loading volume = 100 mL, and elution volume = 7 mL for a clear elution) using 1 piece of surfactant-immobilized membrane. Increasing the membrane volume 5-fold resulted in a better enrichment effectiveness, about 50 fold (loading volume = 500 mL, and elution volume = 10 mL for a clear elution). This evidence proves that scale up is easy for the membrane-type adsorbent and the performance is not deteriorated.
4. Conclusions Strong cation exchange membranes immobilized with octadecyltrimethylammonium hydroxide (or methoxide) surfactants have been successfully prepared and applied to the solid-phase extraction of phenolic compounds from water. The optimal surfactant immobilization procedure requires an HCl prewashing step (for activating the ion exchange groups) and the use of high surfactant feed amount. High enrichment factors and high recoveries were attained by simply using aqueous alcohol solution as the eluant. The whole process time is short and the pressure drop is low, making this method both timesaving and energy-saving. Moreover, scale up can be easily achieved by stacking more membranes together, without any deterioration in the performance. Consequently, this hemimicellar/admicellar surfactant-immobilized membrane design may provide a beneficial alternative for preconcentrating hydrophobic components such as phenolic compounds. It should be noted that other cation exchange membrane materials could also be adopted for the same application.
T.-Y. Wang et al. / Microchemical Journal 96 (2010) 290–295
Acknowledgements The authors are grateful for the financial supports from the National Science Council of Taiwan (Grant No. NSC 97-2221-E-005-041) for Dr. S.-Y. Suen and from the National Science Foundation of the United States (Grant No. OISE-0553369) for Dr. E. D. Conte. Moreover, this work is supported in part by the Ministry of Education, Taiwan under the ATU plan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
N. Li, H.K. Lee, Anal. Chem. 69 (1997) 5193–5199. M.-C. Hennion, J. Chromatogr. A 856 (1999) 3–54. H. Sabik, R. Jeannot, B. Rondeau, J. Chromatogr. A 885 (2000) 217–236. I. Rodriguez, M.P. Llompart, R. Cela, J. Chromatogr. A 885 (2000) 291–304. Y. Cai, G. Jiang, J. Liu, Q. Zhou, Anal. Chem. 75 (2003) 2517–2521. J.K. Autry, E.G. Vaught, E.D. Conte, Microchem. J. 80 (2005) 25–29. Q. Zhao, S.A. Huss, J.K. Autry, K.R. Reid, E.D. Conte, Microchem. J. 80 (2005) 45–54. Q. Zhao, R.T. Philpott, T.D. Oakes, E.D. Conte, Analyst 130 (2005) 750–754. J.J. Kirkland, J.L. Glajch, R.D. Farlee, Anal. Chem. 61 (1989) 2–11. L.E. Sojo, J. Djauhari, J. Chromatogr. A 840 (1999) 21–30. W.P.N. Fernando, M.L. Larrivee, C.F. Poole, Anal. Chem. 65 (1993) 588–595. H. Zou, Q. Luo, D. Zhou, J. Biochem. Biophys. Meth. 49 (2001) 199–240. R. Ghosh, J. Chromatogr. A 952 (2002) 13–27. A. Podgornik, M. Barut, J. Jancar, A. Strancar, T. Tennikova, Anal. Chem. 71 (1999) 2986–2991. [15] T.B. Tennikova, M. Bleha, F. Svec, T.V. Almazova, B.G. Belenkii, J. Chromatogr. 555 (1991) 97–107.
295
[16] M. Kim, K. Saito, S. Furusaki, T. Sato, T. Sugo, I. Ishigaki, J. Chromatogr. 585 (1991) 45–51. [17] T.B. Tennikova, F. Svec, J. Chromatogr. 646 (1993) 279–288. [18] J. Luksa, V. Menart, S. Milicic, B. Kus, V. Gaberc-Porekar, D. Josic, J. Chromatogr. A 661 (1994) 161–168. [19] Q.C. Wang, F. Svec, J.M.J. Frechet, J. Chromatogr. A 669 (1994) 230–235. [20] N. Kubota, M. Kounosu, K. Saito, K. Sugita, K. Watanabe, T. Sugo, J. Chromatogr. A 718 (1995) 27–34. [21] N. Kubota, M. Kounosu, K. Saito, K. Sugita, K. Watanabe, T. Sugo, React. Funct. Polym. 29 (1996) 115–122. [22] N. Kubota, M. Kounosu, K. Saito, K. Sugita, K. Watanabe, T. Sugo, J. Membr. Sci. 134 (1997) 67–73. [23] R. Ghosh, J. Chromatogr. A 923 (2001) 59–64. [24] M.Y. Arica, G.A. Oktem, A. Denizli, Colloids Surf. B 21 (2001) 273–283. [25] G. Bayramoglu, A. Denizli, M.Y. Arica, Polym. Int. 51 (2002) 966–972. [26] J. Danzer, P. Grathwohl, Phys. Chem. Earth 23 (1998) 237–243. [27] T. Saitoh, S. Matsushima, M. Hiraide, J. Chromatogr. A 1040 (2004) 185–191. [28] T. Saitoh, S. Matsushima, M. Hiraide, J. Chromatogr. A 1069 (2005) 271–274. [29] Q. Zhao, J. Simmons, E.D. Conte, J. Chromatogr. A 1132 (2006) 1–7. [30] A. Moral, M.D. Sicilia, S. Rubio, D. Perez-Bendito, Anal. Chim. Acta 608 (2008) 61–72. [31] G.-L. Chen, S.-Y. Suen, S. Vied, K. Pickering, C. Perrin, E.D. Conte, Analyst 134 (2009) 331–336. [32] D. Puig, D. Barceló, J. Chromatogr. A 733 (1996) 371–381. [33] M.A. Osman, G. Seyfang, U.W. Suter, J. Phys. Chem. B 104 (2000) 4433–4439. [34] A. Fan, P. Somasundaran, N.J. Turro, Langmuir 13 (1997) 506–510. [35] H.S. Barud, C.A. Ribeiro, M.S. Crespi, M.A.U. Martines, J. Dexpert-Ghys, R.F.C. Marques, Y. Messaddeq, S.J.L. Ribeiro, J. Therm. Anal. Cal. 87 (2007) 815–818. [36] L. Shen, P.E. Laibinis, T.A. Hatton, Langmuir 15 (1999) 447–453. [37] B. Nikoobakht, M.A. El-Sayed, Langmuir 17 (2001) 6368–6374. [38] M.A. Osman, U.W. Suter, Chem. Mater. 14 (2002) 4408–4415. [39] S.-H. Wu, D.-H. Chen, Chem. Lett. 33 (2004) 406–407.