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Adsorption of phenolic compounds onto trimethylstearylammonium surfactant-immobilized cation-exchange membranes
Microchemical Journal 99 (2011) 388–393
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
Adsorption of phenolic compounds onto trimethylstearylammonium surfactant-immobilized cation-exchange membranes Tien-Yu Wang a, Chih-Hsiung Hsu a, Tzu-Ping Chen a, Eric D. Conte b,⁎, Drew Fenner c, Lisa Crossley c, C. Howie Honeyman c, Shing-Yi Suen a,⁎⁎ a b c
Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky, KY 42101, USA Natrix Separations, Inc., 5295 John Lucas Drive, Unit 6, Burlington, Ontario, Canada L7L 6A8
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Article history: Received 10 June 2011 Accepted 13 June 2011 Available online 2 July 2011 Keywords: Trimethylstearylammonium surfactant Cation-exchange membrane Hemimicelle Admicelle Phenolic compound Adsorption
a b s t r a c t Trimethylstearylammonium hydroxide/methoxide surfactants were immobilized onto two kinds of cationexchange membranes (P81 and S2001) for the adsorption of phenolic compounds in the present study. The results indicate that the membrane with a doubled cation-exchange capacity (S2001) could attain nearly twice of the immobilized surfactant amount, but its surfactant immobilization % was close to or lower than the one with a smaller cation-exchange capacity (P81). By manipulating the feed surfactant concentration, different surfactant arrangements on the membrane surfaces (such as hemimicellar, admicellar, or mixed structure) could be produced. The membranes with theoretically 100% surfactant immobilization revealed the highest hydrophobicity level, and thus they were applied in the batch adsorption of phenolic compounds. According to the batch adsorption results of four phenolic compounds onto the surfactant-immobilized membranes (100% immobilization), the main adsorption mechanism should be hydrophobic interaction and the order of phenolic compound adsorptivity was phenol b 4-nitrophenolb 4-chloro-3-methylphenol ≤ bisphenol A, identical to their log Kow order. Moreover, in a batch adsorption/desorption cycle with 100 mL of 1 ppm bisphenol A aqueous sample tested and 5 mL of 2-propanol as the desorbent, the S2001 membrane (100% surfactant immobilization) could completely recover bisphenol A from water at a 20-fold enrichment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent decades, surfactant-immobilized (-coated or -adsorbed) surfaces and their characteristics have attracted researchers' interest. Alumina [1–6], silica [7–10], and ion-exchange resins [11,12] are the most popular solid surfaces investigated, while the surfactant interaction and the related surface wetting behaviors are focused mainly in the literature. Various mechanisms have been proposed for describing the surfactant adsorption on surface, such as reverse orientation model, bilayer model, small surface micelle model, etc. [13]. According to the reverse orientation model [13], the adsorption isotherm could be divided into four regions. The ionic surfactant molecules are supposed to individually adsorb onto the oppositely-charged surface via electrostatic interaction in region I, and then form surface aggregates and associate into hemimicelles in region II. In both regions, surfactant molecules are oriented with their charged headgroups toward the solid surface and the carbon-chain tails toward the aqueous phase. A ⁎ Corresponding author. ⁎⁎ Corresponding author. Tel.: + 886 4 22852590; fax: + 886 4 22854734. E-mail addresses: [email protected] (E.D. Conte), [email protected] (S.-Y. Suen). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.06.009
hydrophobic monolayer may be completed on the surface. The term hemimicelle represents a monolayer (or head-on orientation) formation of surfactants on surfaces [14]. Next, increasing surfactant concentrations leads to region III, where the second layer starts to build up on the surface due to the hydrophobic interaction between the surfactant carbon chains. Finally, a full bilayer structure is formed in region IV and no more surfactant molecules will be adsorbed. The term admicelle represents the adsorbed micelles formed on surfaces (a bilayer structure) [14]. The surface properties for these four regions are different. Since the surfactant-immobilized surfaces can exhibit special interfacial properties by altering the surface coverage, they can be applied in various processes. A common and practical usage is to employ the surfactant-immobilized surfaces with hydrophobic or multifunctional groups for removing the hydrophobic or other target compounds (e.g. pesticides, polycyclic aromatic hydrocarbons, phenolic compounds, etc.) from an aqueous phase [2–6,8–12]. The surfactant-immobilized adsorbents are usually prepared in particle form which provides a large surface area. An alternative form is the porous membrane [15,16]. Membranes can be easily separated from the liquid solution in a batch process; they can process liquid samples at higher flow rates (or shorter residence times) under low pressure drops and avoid nonuniform packing effects in a flow process [16]; making them superior to particle-
T.-Y. Wang et al. / Microchemical Journal 99 (2011) 388–393
based systems. Our previous studies [15,16] have successfully immobilized cationic and anionic surfactants onto the porous polymeric ionexchange membranes and applied them in the solid-phase extraction of hydrophobic analytes. In the present study, trimethylstearylammonium ions were immobilized onto two cation-exchange membranes which have different negatively-charged groups (phosphate groups and sulfonate groups) and cation-exchange capacities. This investigation included the immobilized surfactant amounts, the possible hemimicellar/admicellar arrangements for the variation in feed surfactant concentration, and the abilities of the resultant surfactant-immobilized membranes to adsorb phenolic compounds. Removing phenolic compounds from water by adsorbents may be difficult due to their varying polarities and acidic properties. For the surfactant-immobilized membranes with the highest level of hydrophobicity, the batch experiments of phenolic compound adsorption were conducted and the results of kinetics, isotherms, and variations in feed volume were compared to explore their possible applications. 2. Experimental 2.1. Materials Two kinds of cation-exchange membranes were adopted in this study: (1) P81 membrane (Whatman, Maidstone, Kent, UK): made of cellulose phosphate, 230 μm thick, with phosphate groups and H + counter ions. (2) S2001 membrane (Natrix Separations, Burlington, Ontario, Canada): made of hydrogels and polypropylene support, 340 μm thick, with sulfonate groups and Na + counter ions. Trimethylstearylammonium chloride, phenol, 4-nitrophenol, 4chloro-3-methylphenol, and bisphenol A were purchased from Sigma-Aldrich (St. Louis, MO, USA) and TCI (Tokyo, Japan). Other chemicals and solvents are of HPLC or analytical grade. 2.2. Cation-exchange capacity measurement After immersion in 20 mL of deionized water for 24 h, one piece of cation-exchange membrane (4 cm2) was soaked in 20 mL of 0.02 M HCl solution for another 24 h. The membrane was then repeatedly washed with deionized water and equilibrated with deionized water for 24 h to remove acid traces. Next, the membrane was equilibrated with 50 mL of 0.01 M NaOH solution for 24 h. The cation-exchange capacity (CEC) was determined from the alkalinity reduction in NaOH solution by back titration using 0.01 M HCl. CEC= (MO,NaOH − ME,NaOH) × V, where MO, NaOH and ME,NaOH are the initial and equilibrium concentrations of NaOH and V is the volume of NaOH solution. 2.3. Trimethylstearylammonium surfactant immobilization Prior to the surfactant immobilization experiment, the chloride ions of trimethylstearylammonium surfactant (C18N +(CH3)3 Cl −) were replaced to the hydroxide/methoxide ions (C18N +(CH3)3 OH −/ OCH3−) through the reaction with Ag2O at a molar ratio of 2:1 in 20 mL of methanol. The reaction products were the surfactant in the hydroxide/methoxide form and white AgCl precipitate. The product solution was filtered by a 0.45 μm cellulose acetate filter to remove the precipitate. According to the results in our previous study (Br − was the counter ion of surfactant) [16], the existence of hydroxide/methoxide ions for surfactants could lead to a significant improvement on surfactant immobilization. The hydroxide/methoxide ions of surfactants exhibited a stronger interaction with the H + counter ions on the membrane, leaving more active positively-charged ammonium groups of surfactants to bind with the negatively-charged groups on the membrane surface. In the prewashing step, one piece of 47 mm cation-exchange membrane disc was rinsed with deionized water for 1 h and then
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immersed in 20 mL of 0.02 M HCl for 24 h. After prewashing, the membrane was rinsed with deionized water again and dried at room temperature for 24 h. In the surfactant immobilization experiment, the prewashed membrane was introduced to the surfactant solution of a certain feed concentration (150, 2000, or 5000 μmol in 20 mL methanol) and the solution was shaken at room temperature for 24 h. After immobilization, the membrane was rinsed with deionized water and then dried at room temperature for 24 h. 2.4. Membrane characterization To check whether the trimethylstearylammonium surfactant was successfully immobilized onto the membrane surface, ATR-FTIR spectra of the prewashed and surfactant-immobilized membranes were measured by Spectrum One, Perkin-Elmer (Wellesley, MA, USA). When the trimethylstearylammonium surfactant was successfully immobilized onto the membrane surface, a water droplet was placed on the membrane and its contact angle was measured using FTA-125, First Ten Angstroms (Portsmouth, VA, USA). Moreover, analyses on the carbon content of the prewashed and surfactant-immobilized 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. 2.5. Batch phenolic compound adsorption experiment A 47 mm disc of trimethylstearylammonium surfactant-immobilized membrane (immobilization percentage = 100%) was placed in a certain volume of phenolic compound solution with a certain feed concentration at room temperature for a certain time. The concentration of phenolic compound in solution was determined by HPLC. Moreover, a batch adsorption/desorption cycle for bisphenol A enrichment was conducted as follows. In the adsorption process, a 47 mm disc of trimethylstearylammonium surfactant-immobilized membrane (immobilization percentage = 100%) was immersed in 100 mL of 1 ppm bisphenol A solution at room temperature for 8 h. The membrane was then placed in 5 mL of 2-propanol for 1 h in order to desorb bisphenol A from the membrane. The bisphenol A concentration in solution was analyzed by HPLC. 2.6. HPLC analysis The HPLC system was comprised of 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. 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. The injection volume was 100 μl. 3. Results and discussion 3.1. Characterization of trimethylstearylammonium surfactantimmobilized cation-exchange membranes Fig. 1 presents the ATR-FTIR results. In comparison with the prewashed membranes, all the surfactant-immobilized membranes revealed the peaks arising from CH2 vibrations at 2850 and 2920 cm − 1[17]. This evidence verified the success of trimethylstearylammonium surfactant (C18N +(CH3)3 OH −/OCH3−) immobilization onto the cation-exchange membranes. Moreover, for both P81 and S2001 membranes, the CH2 peak intensity increased with the higher surfactant amount adopted in the immobilization step. To quantitatively evaluate the effect of feed surfactant concentration on the immobilization performance, the surfactant amount immobilized
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P81 membrane After prewashing 150 mol 2000 mol 5000 mol
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CH2 S2001 membrane After prewashing 150 mol 2000 mol 5000 mol CH2 4000
3000
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Wavenumber(cm ) Fig. 1. ATR-FTIR spectra for the prewashed and surfactant-immobilized membranes. Solution volume = 20 mL.
on the membrane was determined by EA and the data are displayed in Fig. 2. For both types of cation-exchange membranes, an increase in feed surfactant concentration led to a higher degree of surfactant immobilization onto the membrane. It should be noted that the cation-exchange capacity obtained in this study was 150 μeq/47 mm disc for P81 500
P81membrane
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membrane and 340 μeq/disc for S2001 membrane. The cation-exchange capacity of S2001 membrane is twice of the capacity of P81 membrane, thus resulting in the S2001 membrane having a higher surfactant immobilization capacity than P81. To further investigate the occupancy of membrane cationic sites by the surfactant molecules, a term % surfactant immobilization was defined by dividing the immobilized surfactant amount (μmol/disc) by the cation-exchange capacity of membrane (μeq/disc). The % immobilization results are also depicted in Fig. 2. For the feed surfactant amount of 150, 2000, and 5000 μmol (solution volume = 20 mL), the surfactant immobilization % was about 50%, 100%, 150% for P81 membrane and 30%, 100%, 130% for S2001 membrane, respectively. Although the membrane with a larger cationexchange capacity could interact and immobilize more surfactant molecules, the site coverage was close to or even smaller than that of the membrane with a smaller capacity. Based on the above analyses, we may conclude that the key factor influencing the surfactant adsorption capacity is the ion-exchange capacity of membrane, not the type of functional group. The % surfactant immobilization data reveal that the ion-exchange groups on membrane surface could be covered by the surfactant molecules in a single-layer (≤ 100%) or partial bilayer (N 100%) structure. To clearly represent the possible interfacial surfactant arrangements under different immobilization %, a schematic diagram is illustrated in Fig. 3. In the 30–50% immobilization case, the membrane surface would consist partly of C18 chains from the immobilized trimethylstearylammonium surfactants and partly of unutilized negatively-charged groups (sulfonate or phosphate) from the cation-exchange membrane [13,14,16]. The overall membrane surface charge was negative. Theoretically, an entire surfactant monolayer should be formed in the 100% immobilization case and no charges were exposed on the surface (a monolayer hemimicellar structure). As for the 130–150% immobilization case, the excess surfactant molecules build up a second layer through the hydrophobic interaction among their long carbon chains (hemimicellar/admicellar form) [13,14,16]. Thus, the overall membrane surface charge became positive. The contact angle results of water on various membrane surfaces are listed in Fig. 4. It is clearly observed that the water droplet could not stay on the frontal surface of the original P81 and S2001 membranes for any moment. The water droplet directly sank into the porous hydrophilic membrane matrix. After immobilizing the trimethylstearylammonium surfactant molecules, the membrane surface should become more hydrophobic. However, the water droplet did not hold on the frontal surfaces of the surfactantimmobilized P81 membranes in all the cases. This phenomenon could be attributed to the pore configuration of P81 membrane and its lower hydrophobicity. In a different way, the time for the water droplet to submerge into the internal S2001 membrane structure was longer so that the contact angle images of water droplet on the frontal membrane surface could be recorded. In Fig. 4, the contact angle for the case of 2000 μmol feed amount (100% surfactant immobilization) was larger than those for 150 μmol and 5000 μmol feed amount cases and the maximum value was about 110°. This indicates that the frontal membrane surface was most hydrophobic when the cationexchange sites were entirely covered by the surfactant molecules in a monolayer structure. As shown in Fig. 3, the hydrocarbon tails of the trimethylstearylammonium surfactants would be exposed on the surface in 100% immobilization case. When more surfactant molecules were loaded (5000 μmol case, 130–150% immobilization), the admicellar arrangement was formed partly. It resulted in the exposure of positively-charged quaternary ammonium groups of the surfactants, which increased the hydrophilicity of membrane surface. According to the above analyses, the membranes with 100% surfactant immobilization led to the highest level of hydrophobicity. They should be more suitable to apply for hydrophobic compound adsorption.
T.-Y. Wang et al. / Microchemical Journal 99 (2011) 388–393 N
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Fig. 3. Possible surfactant arrangements on membrane surface.
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Table 1 Properties of phenolic compounds. Compound Phenol 4-Nitrophenol 4-Chloro-3-methylphenol Bisphenol A
1.2
P81 membrane
Phenol 4-Nitrophenol 4-Chloro-3-methylphenol Bisphenol A
0.8
C/C0
The membranes with 100% surfactant immobilization were adopted for the adsorption of phenolic compounds. The molecular weight, log Kow, and pKa values for the four selected compounds are presented in Table 1 [10,16,18,19]. The order of hydrophobicity based on their log Kow values is: phenol b 4-nitrophenol b 4-chloro-3methylphenol b bisphenol A. Three kinds of batch adsorption experiments were conducted in this study: adsorption rate tests for different compounds, adsorption equilibrium tests for different compounds, and adsorption rate tests for bisphenol A with different feed volumes. All the experiments were carried out at room temperature. Fig. 5 presents the adsorption rate results of the four phenolic compounds (feed concentration of 100 ppm in 20 mL of water). For all the four compounds, the concentration decreased with time and reached a stable value after ca. 2 h. For the P81 membrane, the adsorption rate order from the slowest to the fastest was phenol b bisphenol A b 4-nitrophenol b 4-chloro-3-methylphenol. However, the equilibrium adsorption percentages for the latter three compounds were almost the same. For the S2001 membrane, the uptake of phenolic compounds followed the order of phenol b 4-nitrophenol b 4chloro-3-methylphenol b bisphenol A, which is identical to their order in hydrophobicity. The above data imply that the main adsorption mechanism between phenolic compounds and the trimethylstearylammonium surfactant-immobilized membranes are hydrophobic interactions. The batch adsorption equilibrium results are depicted in Fig. 6. It should be noted that the solubility of phenolic compound in water depended on its hydrophobicity so that the feed concentrations for 4-
chloro-3-methylphenol and bisphenol A were limited to a lower range. For both P81 and S2001 membranes, the adsorbed amount and the adsorption percentage of phenolic compounds were in the order
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Time (min) Fig. 5. Batch adsorption rate results of various phenolic compounds onto the surfactantimmobilized membranes (100% immobilization). Feed concentration = 100 ppm, solution volume = 20 mL, T = room temperature.
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P81 membrane
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Fig. 6. Batch adsorption equilibrium results of various phenolic compounds onto the surfactant-immobilized membranes (100% immobilization). Solution volume = 20 mL, T = room temperature, equilibrium time = 2 h.
of phenol b 4-nitrophenol b 4-chloro-3-methylphenol ≈ bisphenol A. Again, this order is in good agreement with their hydrophobic strength. It means that the cation-exchange membranes with 100% trimethylstearylammonium surfactant immobilization could effectively adsorb the phenolic compounds via hydrophobic forces. On the other hand, the surfactant-immobilized S2001 membrane could adsorb a greater amount of phenolic compounds. This is attributed to the fact that the cation-exchange capacity of the S2001 membrane was twice to the P81 membrane and hence its immobilized surfactant amount at one disc was doubled (see Fig. 2). In Fig. 6, both the surfactant-immobilized P81 and S2001 membranes adsorbed bisphenol A at high percentages (≥ 95%) under low-medium feed concentrations. Thus, bisphenol A adsorption is further investigated at various feed volumes. One piece of the trimethylstearylammonium surfactant-immobilized membrane disc was immersed in a dilute bisphenol A aqueous solution (1 ppm) and different feed volumes (i.e. different feed amounts) were tested. As presented in Fig. 7, the adsorption rate became slower and the time to reach equilibrium was elongated as the feed volume was raised. At the same feed volume the adsorption percentage of bisphenol A for the S2001 membrane was higher than the P81 membrane, mainly because a larger amount of surfactants had been immobilized on the S2001 membrane. For approximately 100% bisphenol A adsorption, the feed volume had to be b100 mL for the P81 membrane and b300 mL for the S2001 membrane. When one piece of the surfactant-immobilized membrane was incubated in 100 mL of 1 ppm bisphenol A for 8 h, the adsorption percentage (the adsorbed amount/the feed amount × 100%) was 84.3 ± 0.7% for the P81 membrane and 99.6 ± 0.4% for the S2001 membrane. After that, the membrane was placed in 5 mL of 2propanol for 1 h in order to remove the adsorbed bisphenol A out of the membrane. The bisphenol A desorption percentage (the desorbed
amount/the adsorbed amount × 100%) was 102.7 ± 1.0% for P81 and 101.4 ± 0.5% for S2001. A twenty-fold smaller volume of 2-propanol could completely desorb bisphenol A from the trimethylstearylammonium surfactant-immobilized membrane at one-eighth the time period. Consequently, the overall bisphenol A recovery (the desorbed amount/the feed amount) was about 85% for the P81 membrane and 100% for the S2001 membrane. The bisphenol A enrichment achieved by the S2001 membrane at a batch adsorption/desorption cycle was 20-fold, and 17-fold via the P81 membrane.
4. Conclusions This study successfully immobilized trimethylstearylammonium hydroxide/methoxide surfactants onto two kinds of strong cationexchange membranes. The cation-exchange capacity, not the type of anionic functional group, was the main factor affecting the surfactant immobilization capacity. Doubled cation-exchange capacity could attain nearly twice of the immobilized surfactant amount. By controlling the feed surfactant concentration, the resultant membrane could exhibit hemimicellar, admicellar, or mixed structure. The full monolayer surfactant coverage (theoretically 100% surfactant immobilization) revealed the highest hydrophobicity level, and thus the membranes with 100% surfactant immobilization were applied in the batch adsorption of phenolic compounds (especially bisphenol A). The membrane with higher cation-exchange capacity and larger immobilized surfactant amount led to a better bisphenol A adsorption. In a batch adsorption/desorption cycle with dilute bisphenol A aqueous sample tested and 2-propanol as the desorbent, the membrane with a large cation-exchange capacity and 100% surfactant immobilization could completely recover bisphenol A from water and the enrichment was twenty-fold. Conclusively, the surfactant-
T.-Y. Wang et al. / Microchemical Journal 99 (2011) 388–393
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Time (min) Fig. 7. Batch adsorption rate results of bisphenol A onto the surfactant-immobilized membranes (100% immobilization) under various feed volumes. Feed concentration = 1 ppm, T = room temperature.
immobilized membrane may provide a beneficial alternative for concentrating the dilute hydrophobic analytes from water.
Acknowledgements This work is supported in part by the National Science Council (Grant no. NSC 97-2221-E-005-041) and the Ministry of Education (ATU plan), Taiwan, R.O.C. Dr. S.-Y. Suen is grateful for the free flatsheet membrane samples from the Natrix Separations. Dr. E. Conte acknowledges the US National Science Foundation (OISE 0936693).
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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
Adsorption of phenolic compounds onto trimethylstearylammonium surfactant-immobilized cation-exchange membranes Tien-Yu Wang a, Chih-Hsiung Hsu a, Tzu-Ping Chen a, Eric D. Conte b,⁎, Drew Fenner c, Lisa Crossley c, C. Howie Honeyman c, Shing-Yi Suen a,⁎⁎ a b c
Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky, KY 42101, USA Natrix Separations, Inc., 5295 John Lucas Drive, Unit 6, Burlington, Ontario, Canada L7L 6A8
a r t i c l e
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Article history: Received 10 June 2011 Accepted 13 June 2011 Available online 2 July 2011 Keywords: Trimethylstearylammonium surfactant Cation-exchange membrane Hemimicelle Admicelle Phenolic compound Adsorption
a b s t r a c t Trimethylstearylammonium hydroxide/methoxide surfactants were immobilized onto two kinds of cationexchange membranes (P81 and S2001) for the adsorption of phenolic compounds in the present study. The results indicate that the membrane with a doubled cation-exchange capacity (S2001) could attain nearly twice of the immobilized surfactant amount, but its surfactant immobilization % was close to or lower than the one with a smaller cation-exchange capacity (P81). By manipulating the feed surfactant concentration, different surfactant arrangements on the membrane surfaces (such as hemimicellar, admicellar, or mixed structure) could be produced. The membranes with theoretically 100% surfactant immobilization revealed the highest hydrophobicity level, and thus they were applied in the batch adsorption of phenolic compounds. According to the batch adsorption results of four phenolic compounds onto the surfactant-immobilized membranes (100% immobilization), the main adsorption mechanism should be hydrophobic interaction and the order of phenolic compound adsorptivity was phenol b 4-nitrophenolb 4-chloro-3-methylphenol ≤ bisphenol A, identical to their log Kow order. Moreover, in a batch adsorption/desorption cycle with 100 mL of 1 ppm bisphenol A aqueous sample tested and 5 mL of 2-propanol as the desorbent, the S2001 membrane (100% surfactant immobilization) could completely recover bisphenol A from water at a 20-fold enrichment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent decades, surfactant-immobilized (-coated or -adsorbed) surfaces and their characteristics have attracted researchers' interest. Alumina [1–6], silica [7–10], and ion-exchange resins [11,12] are the most popular solid surfaces investigated, while the surfactant interaction and the related surface wetting behaviors are focused mainly in the literature. Various mechanisms have been proposed for describing the surfactant adsorption on surface, such as reverse orientation model, bilayer model, small surface micelle model, etc. [13]. According to the reverse orientation model [13], the adsorption isotherm could be divided into four regions. The ionic surfactant molecules are supposed to individually adsorb onto the oppositely-charged surface via electrostatic interaction in region I, and then form surface aggregates and associate into hemimicelles in region II. In both regions, surfactant molecules are oriented with their charged headgroups toward the solid surface and the carbon-chain tails toward the aqueous phase. A ⁎ Corresponding author. ⁎⁎ Corresponding author. Tel.: + 886 4 22852590; fax: + 886 4 22854734. E-mail addresses: [email protected] (E.D. Conte), [email protected] (S.-Y. Suen). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.06.009
hydrophobic monolayer may be completed on the surface. The term hemimicelle represents a monolayer (or head-on orientation) formation of surfactants on surfaces [14]. Next, increasing surfactant concentrations leads to region III, where the second layer starts to build up on the surface due to the hydrophobic interaction between the surfactant carbon chains. Finally, a full bilayer structure is formed in region IV and no more surfactant molecules will be adsorbed. The term admicelle represents the adsorbed micelles formed on surfaces (a bilayer structure) [14]. The surface properties for these four regions are different. Since the surfactant-immobilized surfaces can exhibit special interfacial properties by altering the surface coverage, they can be applied in various processes. A common and practical usage is to employ the surfactant-immobilized surfaces with hydrophobic or multifunctional groups for removing the hydrophobic or other target compounds (e.g. pesticides, polycyclic aromatic hydrocarbons, phenolic compounds, etc.) from an aqueous phase [2–6,8–12]. The surfactant-immobilized adsorbents are usually prepared in particle form which provides a large surface area. An alternative form is the porous membrane [15,16]. Membranes can be easily separated from the liquid solution in a batch process; they can process liquid samples at higher flow rates (or shorter residence times) under low pressure drops and avoid nonuniform packing effects in a flow process [16]; making them superior to particle-
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based systems. Our previous studies [15,16] have successfully immobilized cationic and anionic surfactants onto the porous polymeric ionexchange membranes and applied them in the solid-phase extraction of hydrophobic analytes. In the present study, trimethylstearylammonium ions were immobilized onto two cation-exchange membranes which have different negatively-charged groups (phosphate groups and sulfonate groups) and cation-exchange capacities. This investigation included the immobilized surfactant amounts, the possible hemimicellar/admicellar arrangements for the variation in feed surfactant concentration, and the abilities of the resultant surfactant-immobilized membranes to adsorb phenolic compounds. Removing phenolic compounds from water by adsorbents may be difficult due to their varying polarities and acidic properties. For the surfactant-immobilized membranes with the highest level of hydrophobicity, the batch experiments of phenolic compound adsorption were conducted and the results of kinetics, isotherms, and variations in feed volume were compared to explore their possible applications. 2. Experimental 2.1. Materials Two kinds of cation-exchange membranes were adopted in this study: (1) P81 membrane (Whatman, Maidstone, Kent, UK): made of cellulose phosphate, 230 μm thick, with phosphate groups and H + counter ions. (2) S2001 membrane (Natrix Separations, Burlington, Ontario, Canada): made of hydrogels and polypropylene support, 340 μm thick, with sulfonate groups and Na + counter ions. Trimethylstearylammonium chloride, phenol, 4-nitrophenol, 4chloro-3-methylphenol, and bisphenol A were purchased from Sigma-Aldrich (St. Louis, MO, USA) and TCI (Tokyo, Japan). Other chemicals and solvents are of HPLC or analytical grade. 2.2. Cation-exchange capacity measurement After immersion in 20 mL of deionized water for 24 h, one piece of cation-exchange membrane (4 cm2) was soaked in 20 mL of 0.02 M HCl solution for another 24 h. The membrane was then repeatedly washed with deionized water and equilibrated with deionized water for 24 h to remove acid traces. Next, the membrane was equilibrated with 50 mL of 0.01 M NaOH solution for 24 h. The cation-exchange capacity (CEC) was determined from the alkalinity reduction in NaOH solution by back titration using 0.01 M HCl. CEC= (MO,NaOH − ME,NaOH) × V, where MO, NaOH and ME,NaOH are the initial and equilibrium concentrations of NaOH and V is the volume of NaOH solution. 2.3. Trimethylstearylammonium surfactant immobilization Prior to the surfactant immobilization experiment, the chloride ions of trimethylstearylammonium surfactant (C18N +(CH3)3 Cl −) were replaced to the hydroxide/methoxide ions (C18N +(CH3)3 OH −/ OCH3−) through the reaction with Ag2O at a molar ratio of 2:1 in 20 mL of methanol. The reaction products were the surfactant in the hydroxide/methoxide form and white AgCl precipitate. The product solution was filtered by a 0.45 μm cellulose acetate filter to remove the precipitate. According to the results in our previous study (Br − was the counter ion of surfactant) [16], the existence of hydroxide/methoxide ions for surfactants could lead to a significant improvement on surfactant immobilization. The hydroxide/methoxide ions of surfactants exhibited a stronger interaction with the H + counter ions on the membrane, leaving more active positively-charged ammonium groups of surfactants to bind with the negatively-charged groups on the membrane surface. In the prewashing step, one piece of 47 mm cation-exchange membrane disc was rinsed with deionized water for 1 h and then
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immersed in 20 mL of 0.02 M HCl for 24 h. After prewashing, the membrane was rinsed with deionized water again and dried at room temperature for 24 h. In the surfactant immobilization experiment, the prewashed membrane was introduced to the surfactant solution of a certain feed concentration (150, 2000, or 5000 μmol in 20 mL methanol) and the solution was shaken at room temperature for 24 h. After immobilization, the membrane was rinsed with deionized water and then dried at room temperature for 24 h. 2.4. Membrane characterization To check whether the trimethylstearylammonium surfactant was successfully immobilized onto the membrane surface, ATR-FTIR spectra of the prewashed and surfactant-immobilized membranes were measured by Spectrum One, Perkin-Elmer (Wellesley, MA, USA). When the trimethylstearylammonium surfactant was successfully immobilized onto the membrane surface, a water droplet was placed on the membrane and its contact angle was measured using FTA-125, First Ten Angstroms (Portsmouth, VA, USA). Moreover, analyses on the carbon content of the prewashed and surfactant-immobilized 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. 2.5. Batch phenolic compound adsorption experiment A 47 mm disc of trimethylstearylammonium surfactant-immobilized membrane (immobilization percentage = 100%) was placed in a certain volume of phenolic compound solution with a certain feed concentration at room temperature for a certain time. The concentration of phenolic compound in solution was determined by HPLC. Moreover, a batch adsorption/desorption cycle for bisphenol A enrichment was conducted as follows. In the adsorption process, a 47 mm disc of trimethylstearylammonium surfactant-immobilized membrane (immobilization percentage = 100%) was immersed in 100 mL of 1 ppm bisphenol A solution at room temperature for 8 h. The membrane was then placed in 5 mL of 2-propanol for 1 h in order to desorb bisphenol A from the membrane. The bisphenol A concentration in solution was analyzed by HPLC. 2.6. HPLC analysis The HPLC system was comprised of 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. 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. The injection volume was 100 μl. 3. Results and discussion 3.1. Characterization of trimethylstearylammonium surfactantimmobilized cation-exchange membranes Fig. 1 presents the ATR-FTIR results. In comparison with the prewashed membranes, all the surfactant-immobilized membranes revealed the peaks arising from CH2 vibrations at 2850 and 2920 cm − 1[17]. This evidence verified the success of trimethylstearylammonium surfactant (C18N +(CH3)3 OH −/OCH3−) immobilization onto the cation-exchange membranes. Moreover, for both P81 and S2001 membranes, the CH2 peak intensity increased with the higher surfactant amount adopted in the immobilization step. To quantitatively evaluate the effect of feed surfactant concentration on the immobilization performance, the surfactant amount immobilized
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P81 membrane After prewashing 150 mol 2000 mol 5000 mol
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CH2 S2001 membrane After prewashing 150 mol 2000 mol 5000 mol CH2 4000
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Wavenumber(cm ) Fig. 1. ATR-FTIR spectra for the prewashed and surfactant-immobilized membranes. Solution volume = 20 mL.
on the membrane was determined by EA and the data are displayed in Fig. 2. For both types of cation-exchange membranes, an increase in feed surfactant concentration led to a higher degree of surfactant immobilization onto the membrane. It should be noted that the cation-exchange capacity obtained in this study was 150 μeq/47 mm disc for P81 500
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membrane and 340 μeq/disc for S2001 membrane. The cation-exchange capacity of S2001 membrane is twice of the capacity of P81 membrane, thus resulting in the S2001 membrane having a higher surfactant immobilization capacity than P81. To further investigate the occupancy of membrane cationic sites by the surfactant molecules, a term % surfactant immobilization was defined by dividing the immobilized surfactant amount (μmol/disc) by the cation-exchange capacity of membrane (μeq/disc). The % immobilization results are also depicted in Fig. 2. For the feed surfactant amount of 150, 2000, and 5000 μmol (solution volume = 20 mL), the surfactant immobilization % was about 50%, 100%, 150% for P81 membrane and 30%, 100%, 130% for S2001 membrane, respectively. Although the membrane with a larger cationexchange capacity could interact and immobilize more surfactant molecules, the site coverage was close to or even smaller than that of the membrane with a smaller capacity. Based on the above analyses, we may conclude that the key factor influencing the surfactant adsorption capacity is the ion-exchange capacity of membrane, not the type of functional group. The % surfactant immobilization data reveal that the ion-exchange groups on membrane surface could be covered by the surfactant molecules in a single-layer (≤ 100%) or partial bilayer (N 100%) structure. To clearly represent the possible interfacial surfactant arrangements under different immobilization %, a schematic diagram is illustrated in Fig. 3. In the 30–50% immobilization case, the membrane surface would consist partly of C18 chains from the immobilized trimethylstearylammonium surfactants and partly of unutilized negatively-charged groups (sulfonate or phosphate) from the cation-exchange membrane [13,14,16]. The overall membrane surface charge was negative. Theoretically, an entire surfactant monolayer should be formed in the 100% immobilization case and no charges were exposed on the surface (a monolayer hemimicellar structure). As for the 130–150% immobilization case, the excess surfactant molecules build up a second layer through the hydrophobic interaction among their long carbon chains (hemimicellar/admicellar form) [13,14,16]. Thus, the overall membrane surface charge became positive. The contact angle results of water on various membrane surfaces are listed in Fig. 4. It is clearly observed that the water droplet could not stay on the frontal surface of the original P81 and S2001 membranes for any moment. The water droplet directly sank into the porous hydrophilic membrane matrix. After immobilizing the trimethylstearylammonium surfactant molecules, the membrane surface should become more hydrophobic. However, the water droplet did not hold on the frontal surfaces of the surfactantimmobilized P81 membranes in all the cases. This phenomenon could be attributed to the pore configuration of P81 membrane and its lower hydrophobicity. In a different way, the time for the water droplet to submerge into the internal S2001 membrane structure was longer so that the contact angle images of water droplet on the frontal membrane surface could be recorded. In Fig. 4, the contact angle for the case of 2000 μmol feed amount (100% surfactant immobilization) was larger than those for 150 μmol and 5000 μmol feed amount cases and the maximum value was about 110°. This indicates that the frontal membrane surface was most hydrophobic when the cationexchange sites were entirely covered by the surfactant molecules in a monolayer structure. As shown in Fig. 3, the hydrocarbon tails of the trimethylstearylammonium surfactants would be exposed on the surface in 100% immobilization case. When more surfactant molecules were loaded (5000 μmol case, 130–150% immobilization), the admicellar arrangement was formed partly. It resulted in the exposure of positively-charged quaternary ammonium groups of the surfactants, which increased the hydrophilicity of membrane surface. According to the above analyses, the membranes with 100% surfactant immobilization led to the highest level of hydrophobicity. They should be more suitable to apply for hydrophobic compound adsorption.
T.-Y. Wang et al. / Microchemical Journal 99 (2011) 388–393 N
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Table 1 Properties of phenolic compounds. Compound Phenol 4-Nitrophenol 4-Chloro-3-methylphenol Bisphenol A
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Phenol 4-Nitrophenol 4-Chloro-3-methylphenol Bisphenol A
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The membranes with 100% surfactant immobilization were adopted for the adsorption of phenolic compounds. The molecular weight, log Kow, and pKa values for the four selected compounds are presented in Table 1 [10,16,18,19]. The order of hydrophobicity based on their log Kow values is: phenol b 4-nitrophenol b 4-chloro-3methylphenol b bisphenol A. Three kinds of batch adsorption experiments were conducted in this study: adsorption rate tests for different compounds, adsorption equilibrium tests for different compounds, and adsorption rate tests for bisphenol A with different feed volumes. All the experiments were carried out at room temperature. Fig. 5 presents the adsorption rate results of the four phenolic compounds (feed concentration of 100 ppm in 20 mL of water). For all the four compounds, the concentration decreased with time and reached a stable value after ca. 2 h. For the P81 membrane, the adsorption rate order from the slowest to the fastest was phenol b bisphenol A b 4-nitrophenol b 4-chloro-3-methylphenol. However, the equilibrium adsorption percentages for the latter three compounds were almost the same. For the S2001 membrane, the uptake of phenolic compounds followed the order of phenol b 4-nitrophenol b 4chloro-3-methylphenol b bisphenol A, which is identical to their order in hydrophobicity. The above data imply that the main adsorption mechanism between phenolic compounds and the trimethylstearylammonium surfactant-immobilized membranes are hydrophobic interactions. The batch adsorption equilibrium results are depicted in Fig. 6. It should be noted that the solubility of phenolic compound in water depended on its hydrophobicity so that the feed concentrations for 4-
chloro-3-methylphenol and bisphenol A were limited to a lower range. For both P81 and S2001 membranes, the adsorbed amount and the adsorption percentage of phenolic compounds were in the order
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Time (min) Fig. 5. Batch adsorption rate results of various phenolic compounds onto the surfactantimmobilized membranes (100% immobilization). Feed concentration = 100 ppm, solution volume = 20 mL, T = room temperature.
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Fig. 6. Batch adsorption equilibrium results of various phenolic compounds onto the surfactant-immobilized membranes (100% immobilization). Solution volume = 20 mL, T = room temperature, equilibrium time = 2 h.
of phenol b 4-nitrophenol b 4-chloro-3-methylphenol ≈ bisphenol A. Again, this order is in good agreement with their hydrophobic strength. It means that the cation-exchange membranes with 100% trimethylstearylammonium surfactant immobilization could effectively adsorb the phenolic compounds via hydrophobic forces. On the other hand, the surfactant-immobilized S2001 membrane could adsorb a greater amount of phenolic compounds. This is attributed to the fact that the cation-exchange capacity of the S2001 membrane was twice to the P81 membrane and hence its immobilized surfactant amount at one disc was doubled (see Fig. 2). In Fig. 6, both the surfactant-immobilized P81 and S2001 membranes adsorbed bisphenol A at high percentages (≥ 95%) under low-medium feed concentrations. Thus, bisphenol A adsorption is further investigated at various feed volumes. One piece of the trimethylstearylammonium surfactant-immobilized membrane disc was immersed in a dilute bisphenol A aqueous solution (1 ppm) and different feed volumes (i.e. different feed amounts) were tested. As presented in Fig. 7, the adsorption rate became slower and the time to reach equilibrium was elongated as the feed volume was raised. At the same feed volume the adsorption percentage of bisphenol A for the S2001 membrane was higher than the P81 membrane, mainly because a larger amount of surfactants had been immobilized on the S2001 membrane. For approximately 100% bisphenol A adsorption, the feed volume had to be b100 mL for the P81 membrane and b300 mL for the S2001 membrane. When one piece of the surfactant-immobilized membrane was incubated in 100 mL of 1 ppm bisphenol A for 8 h, the adsorption percentage (the adsorbed amount/the feed amount × 100%) was 84.3 ± 0.7% for the P81 membrane and 99.6 ± 0.4% for the S2001 membrane. After that, the membrane was placed in 5 mL of 2propanol for 1 h in order to remove the adsorbed bisphenol A out of the membrane. The bisphenol A desorption percentage (the desorbed
amount/the adsorbed amount × 100%) was 102.7 ± 1.0% for P81 and 101.4 ± 0.5% for S2001. A twenty-fold smaller volume of 2-propanol could completely desorb bisphenol A from the trimethylstearylammonium surfactant-immobilized membrane at one-eighth the time period. Consequently, the overall bisphenol A recovery (the desorbed amount/the feed amount) was about 85% for the P81 membrane and 100% for the S2001 membrane. The bisphenol A enrichment achieved by the S2001 membrane at a batch adsorption/desorption cycle was 20-fold, and 17-fold via the P81 membrane.
4. Conclusions This study successfully immobilized trimethylstearylammonium hydroxide/methoxide surfactants onto two kinds of strong cationexchange membranes. The cation-exchange capacity, not the type of anionic functional group, was the main factor affecting the surfactant immobilization capacity. Doubled cation-exchange capacity could attain nearly twice of the immobilized surfactant amount. By controlling the feed surfactant concentration, the resultant membrane could exhibit hemimicellar, admicellar, or mixed structure. The full monolayer surfactant coverage (theoretically 100% surfactant immobilization) revealed the highest hydrophobicity level, and thus the membranes with 100% surfactant immobilization were applied in the batch adsorption of phenolic compounds (especially bisphenol A). The membrane with higher cation-exchange capacity and larger immobilized surfactant amount led to a better bisphenol A adsorption. In a batch adsorption/desorption cycle with dilute bisphenol A aqueous sample tested and 2-propanol as the desorbent, the membrane with a large cation-exchange capacity and 100% surfactant immobilization could completely recover bisphenol A from water and the enrichment was twenty-fold. Conclusively, the surfactant-
T.-Y. Wang et al. / Microchemical Journal 99 (2011) 388–393
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Time (min) Fig. 7. Batch adsorption rate results of bisphenol A onto the surfactant-immobilized membranes (100% immobilization) under various feed volumes. Feed concentration = 1 ppm, T = room temperature.
immobilized membrane may provide a beneficial alternative for concentrating the dilute hydrophobic analytes from water.
Acknowledgements This work is supported in part by the National Science Council (Grant no. NSC 97-2221-E-005-041) and the Ministry of Education (ATU plan), Taiwan, R.O.C. Dr. S.-Y. Suen is grateful for the free flatsheet membrane samples from the Natrix Separations. Dr. E. Conte acknowledges the US National Science Foundation (OISE 0936693).
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