- Email: [email protected]
Estrogenic compounds removal by fullerene-containing membranes
Desalination 214 (2007) 83–90
Estrogenic compounds removal by fullerene-containing membranes X. Jina, J.Y. Hua*, M.L. Tinta, S.L. Onga, Y. Biryulinb, G. Polotskayac a
Center for Water Research, Division of Environmental Science and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Tel. +65 65164540; Fax: +65 67744202; email: [email protected] b Ioffe Physico-Technical Institute, Russian Academy of Sciences, Polytekhnicheskaya 26, St. Petersburg 194021, Russia c Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy 31, St. Petersburg 199004, Russia Received 24 January 2006; Accepted 15 October 2006
Abstract Estrogenic pollutants present in surface water and treated wastewater could exert significant effects on biota exposed in the receiving environment. Amongst them, the impacts of steroid estrogens such as estrone, estradiol and ethinylestradiol are prominent as they have the highest endocrine disrupting potency than other synthetic endocrine disrupting chemicals (EDCs), despite of low concentration. Since these steroid estrogens are hydrophobic compounds of low volatility, adsorption plays an important role in their removal. Therefore, hydrophobic polymer membranes and strong adsorbent could be effective in removal of estrogens from aqueous phase. In this study, new polymer membranes based on hydrophobic polymer – poly (2, 6-dimethyl-1, 4-phenylene oxide) (PPO) modified by fullerene C60 were studied on removal and adsorptive behaviors of estrogenic compounds. The removal, adsorption rate and capacity of estrone by membranes with different fullerene compositions (PPO; 2%wtC60-PPO; 10%wtC60-PPO) through dead-end filtration and static adsorption experiments were investigated. SEM images demonstrated increase of pore size and porosity in top layer of fullerene-containing membranes compared to pure PPO membrane. Moreover, cross-section of PPO membrane had sponge–like structure while membranes containing C60 (2% and 10%) had finger-like structure. As a result, the permeate flux increased with the increased percentage of fullerene in membrane. However the removal of estrone by PPO membrane with 10% fullerene did not decrease obviously compared with the one without fullerene although the flux increased by about eight times. It was observed that all of the three kinds of membrane showed very good removal of estrone (more than 96%). Results for long-term filtration showed that the 10%wtC60-PPO membrane was able to maintain its excellent removal performance of at least 95%. The results from the static adsorption tests showed that after 12 h, all membranes reached ultimate adsorption of estrone with a similar value of adsorption capacity, although PPO membrane with 2% C60 had the fastest rate of adsorption, followed by PPO membrane with 10% C60 and PPO membrane before reaching the plateau value. This can probably be explained by their pore size on membrane surface and internal structure. *Corresponding author. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.10.019
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Keywords: Estrogens; Adsorption; Fullerene; Membrane; Water reuse, Purification
1. Introduction Water reclamation is an alternative way to augment drinking water resource. However, new emerging contaminants in source water have been heightening the public health concerns. Estrogenic compounds, which mimic or partially mimic the sex steroid hormone estrogens, are the endocrine disruptors mainly concerned in water reclamation. Amongst many kinds of estrogenic compounds, the impact of steroid estrogens such as estrone, estradiol and ethinylestradiol are prominent due to their much higher endocrine disrupting potency than others [1–3]. Estrogenic pollutants present in seawater, surface water and treated wastewater could exert significant effects on biota exposed in the receiving environment. Due to many relative evidences have been observed, estrogenic compounds are believed to affect human reproductive system via food and drinking water. These effects include the development of hormone-dependent cancers, disorders of the reproductive tract, decreased levels of sperm production and compromised reproductive fitness [4]. Because of very low concentration in the range of 1–30 ng/L, the ability of conventional wastewater treatment to remove trace level of estrogenic compounds is limited. Hence, advance technological treatment is essential for more stringent requirements for drinking water and water reclamation. Membrane technology with high selectivity have been more and more frequently applied in water treatment processes due to its operational simplicity, modular and compact equipment comparing to physical and chemical counterparts. In recent years, several researchers have shown that nanofiltration and reverse osmosis are capable of removing trace organic compounds including natural hormones and a wide range of pesticides [5,6]. Due to the
interaction of solution with membrane polymeric matrix, retention can be influenced by both membrane and solution characteristics. Since steroid hormones are hydrophobic organic compounds of low volatility, the mechanism of sorption is expected to play a vital role in their removal during the membrane process. Therefore, membrane material, being hydrophobic in practical usages, which has a relatively high adsorption capacity to achieve a stable and effective removal performance of them, is highly preferred. However, care also must be taken when evaluating the performance of a membrane as desorption may lead to a high risk of bulk release of these trace contaminants. Thus special treatment of membrane is needed for better removal of trace estrogenic compounds. With the rapid development of fullerene science and technology, fullerene-containing polymers have been incorporated into membrane technology since 1992. A series of studies has been shown that polymer membranes modified by fullerene C60 substantially improve their initial properties [7]. However, fullerene-containing polymers were mainly involved in the application of gas separation [7,8]. Moreover, PPO is known as a hydrophobic material having good physicochemical properties. Some results on study of adsorption properties of fullerenes have been presented [9]. Fullerenes as adsorbents of organic compounds in water are much more effective than soot or activated carbon. Adsorption ability of fullerene is realized mainly through dispersion interactions. It has been shown that combination of PPO and С60 molecules gives a donor-acceptor complex. Therefore, it might be a prospect for using fullerene-containing PPO membranes to reject natural steroid estrogens from wastewater. In this study, new asymmetric membranes based on hydrophobic polymer PPO modified by
X. Jin et al. / Desalination 214 (2007) 83–90
fullerene C60 were studied on their ability to adsorb and remove natural hormone estrone. The purposes of this study were (1) to investigate the adsorption of natural hormone estrone on fullerene-containing PPO membranes and (2) to understand the removal performance of fullerenecontaining PPO membranes on estrone removal. 2. Materials and methods 2.1. Membranes Asymmetric polymer membranes based on PPO and its compositions containing 2 and 10%wt C60 were used in this study. The fullerenecontaining compositions containing were obtained by solid-phase interaction of C60 and PPO. Asymmetric membranes were prepared by casting a 6%wt polymer solution containing a PPO or C60-PPO composition in a mixed chloroform: butanol (85:15, %wt) solvent on a glass plate which was immersed in a coagulation bath with ethanol. After 20 h precipitation, membranes were transferred into a bath with 40%wt glycerol in aqueous solution and dried in air. The membranes were washed in distilled water prior to adsorption and filtration tests. The incorporation of fullerene into PPO membrane was accompanied by a color change of the membrane from pure white to brown. 2.2. Membrane removal experiments Membrane removal experiments were conducted by using stirrer cell membrane filtration unit (Millipore, model series 8200, USA) with an 800ml reservoir. 150 ml of sample water with initial estrone concentration of 5 µg/l was placed into the stirrer cell reactor and filtration was conducted at 2 bar pressure. After 100 ml of permeate was collected, filtration was stopped and 50 ml of water remaining in the stirrer cell was collected as retentate. Effective membrane surface area was 28.7 cm2. A magnetic stirrer was used to minimize polarization concentration
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effects and the stirred speed was fixed at 200 rpm. Instrumental grade nitrogen was used to pressurize the stirred cell. A new membrane was used for each experiment. For long-term filtration, six permeate samples, each of a volume of 100 ml were collected and hence a total of 600 ml of the feed solution was filtered. Parameter used to quantify the membrane removal efficiency was the estrone removal R:
⎛ Cp ⎞ R = 100% × ⎜ 1 − ⎟ ⎝ C0 ⎠
(1)
where Cp was the concentration of estrone in permeate and C0 was the initial concentration of estrone in the stirred cell. 2.3. Static adsorption experiments To compare the adsorption capacity and rate of membrane with different fullerene compositions, static adsorption tests were conducted. Pieces of membrane having total surface area of 20 cm2 were put into 500 ml flask containing 500 ml of sample water with initial estrone concentration of 5 µg/l. The flask was then shaken at 150 rpm to ensure a homogeneous solution. At stipulated timings, 40 ml of sample solution was collected. The concentration of estrone in the sample was monitored to calculate the amount of estrone adsorbed on the membranes. 2.4. Chemicals and solution chemistry Estrone was purchased from Sigma Aldrich (Singapore). Its characteristics have been described elsewhere [1,10]. Diameter of estrone molecule is estimated to be about 0.8 nm using Stokes-Einstein equation. The acid dissociation constant, pKa, of estrone is 10.4. Octanol-water partitioning coefficient (logKow) is 3.43. The background electrolyte of sample water consisted of 1 mM NaHCO3 and 8 mM NaCl and pH was adjusted to 7 with 1N HCl.
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2.5. Estrogen detection
3. Results and discussion
In this study, estrone was extracted from an aqueous sample by solid phase extraction (SPE) cartridge filled with 500mg of C18 (55 µm) from Phenomenex, USA. Before the analytes were extracted, C18 cartridge was conditioned by 7 ml of acetone, 5 ml of methanol and 10 ml of ultrapure water subsequently. The sample water pH was lowered to 3 by using acetic acid before loading through the C18 cartridge at flow rate of 3 ml/min with the aid of a vacuum pump. After sample loading, the cartridge was eluted with 5 ml of acetone. Analytes were re-extracted from the cartridge by passing through another 5 ml of acetone. The final eluant was collected in a brown glass vial and was allowed to dry under a gentle stream of nitrogen. After drying, 0.4 ml of methanol was added into the vial. Then concentrations of estrone in methanol after SPE pretreatment were measured by LC/MS/MS (MDS Sciex API 2000 tandem triple-quadropole mass spectrometer) from Applied Biosystems, USA. Analytes were chromatographed on a 2.1×150 mm column filled with 3.5 µm C18 reversed phase packing (Agilent, USA). High purity nitrogen gas was used as nebulizer, drying, curtain and collision gases. The setting for curtain gas was 40 psi. For estrone, the declustering potential was 30 V. MRM mode was chosen for quantification. The ion spay voltage was !4500 V and the probe temperature was 450EC. All the source and instrument parameters for monitoring estrone were optimized by standard solutions by a syringe pump. After observing collision-induced dissociation (CID) spectra obtained by full-scan production experiments, the following MRM pairs were chosen: 269.2/145.0; 269.2/143.0.
3.1. Analysis of membrane structure
2.6. Membrane structure analysis Asymmetric membrane structure analyses were performed using a scanning electron microscope (SEM), Phillips XL30 FEG SEM, in this study.
Asymmetric membranes structure was showed by scanning electron microscopy. Fig. 1 demonstrates SEM micrographs of skin surface and the cross-section of PPO and 10 wt%C60-PPO membranes. All membranes consisted of a porous support and a dense top layer which also contained pores. The structures of PPO and C60-PPO membranes differed greatly. The micrograph of PPO top layer shows that the skin surface was rather smooth; surface pores were dead-end with thin partitions and similar thin partitions formed a sponge-like porous structure throughout the cross section. As for 10%wt С60-PPO membrane, the skin surface was sufficiently smooth but inhomogeneous. Surface pores were also deadend but deeper. The cross section of 10%wt C60PPO (just as that of 2%wt C60-PPO) membrane demonstrates the finger-like structure. A difference can be seen in the thickness of PPO and C60PPO membranes prepared in the same conditions. Furthermore, pore size and porosity of a dense top layer are higher in the case of C60-PPO. Physical cross-linking of several PPO chains by C60 molecules leads to increasing microheterogeneity of the system in solution and favours the formation of more developed porous structure of fullerene-containing membranes [11]. 3.2. Adsorption of estrone on fullerenecontaining PPO membranes The adsorption percentages of estrone on membrane with different composition of fullerene at static adsorption condition are shown in Fig. 2. The dependence of adsorption capacity on time demonstrates high adsorption activity of both PPO and fullerene-containing PPO membranes with respect to estrone. It was observed that after 12 h, adsorption of estrone on the three kinds of membrane had attained a plateau value and the ultimate adsorption extents by all membranes
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Fig. 1. SEM images of skin surface and cross-section for PPO and 10 wt%C60-PPO membranes.
Fig. 2. Adsorption of estrone on membrane with different composition of fullerene.
were similar. We attempt to supply an explanation of these observed results. The adsorption of estrone molecules on membrane depends on the following facts: size of contact area in the membrane and the presence of C60 molecules that is known as lipophilic agent which can give additional sites in the membrane for estrone adsorption. It is evident that the amount of adsorption sites depends on the contact area. Porous structure of PPO and C60-PPO membranes differs greatly. Different porosity implies different internal area that is available for adsorption. Moreover, the PPO membrane exhibits the largest internal area due to its sponge-like porous structure. On the other hand, membranes containing
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2 wt% and 10 wt% C60 have additional adsorption sites due to the fact that C60 molecules are active adsorbents but their finger-like internal surfaces possess smaller area than that of PPO membrane. As a result of the opposition of these two factors, the three kinds of membrane demonstrated similar adsorption capacity of estrone at equilibrium state. In addition, it was found that PPO membrane with 2 wt% C60 had the fastest rate of adsorption, followed by PPO membrane with 10 wt% C60 and pure PPO membrane before reaching the plateau value. The reason seems to be due to the difference in their pore size on surface and internal structure. For adsorption rate, one of the influencing factors could be diffusion. The larger the pore size, the quicker the diffusion of estrone from the proximity of the membrane surface into membrane internal porous structure. Based on SEM images (Fig. 1), C60-PPO membrane has bigger pore size on the surface than that of the PPO membrane. On the other hand, the PPO membrane showed bigger internal area than that of C60-PPO membrane. Once estrone molecules enter into membrane internal structure, bigger internal area of membrane may lead to better diffusion of estrone inside the membrane. Combining these two opposite factors, an optimum in terms of adsorption rate was realized in 2 wt% C60-PPO membrane. 3.3. Estrone removal by membranes with different compositions of fullerene Table 1 shows the data on flux and estrone removal in filtration experiments with ultrapure water and aqueous solution of estrone by using PPO membranes containing different C60 content at 2 bar. It can be seen clearly that fullerene incorporation improved the pure water flux by 8 times with the increased percentage of fullerene in membrane that indicated modifying PPO with fullerene was potentially useful for improving the transport properties of the membrane. This
Table 1 Flux and estrone removal in filtration tests Membrane
PPO 2 wt%C60 PPO 10 wt%C60 PPO
Pure water flux (×105 cm/s)
Filtration of estrone solution Permeate flux (×105 cm/s)
Estrone removal (%)
33.6 60.6
22.8 39.2
97.2 98.8
273.0
193.6
96.8
phenomenon was not surprising since fullerene incorporation lead to the increases in membrane surface pore size as mentioned earlier. It was also observed that fullerene incorporation improved the permeate flux by around 8 times with increased percentage of fullerene in membrane during filtration of aqueous solution of estrone. It was, however, interesting to note that estrone removal by 10 wt%C60+PPO membrane was similar with the one without fullerene. It was shown in the dead-end filtration tests that all of the three kinds of membrane provided very good removal of estrone (more than 96%). Membranes potentially useful in organic retention should possess high permeability and high removal. However, for most membrane materials these two properties are antipathetic. Based on the results shown above, 10 wt%C60-PPO membrane has an optimum combination of permeability and removal and would have a prospective in practical application for estrone removal comparing with the original PPO membrane. In membrane technology, there are three main mechanisms in which contaminants can be retained. They are namely charge repulsion, steric hindrance and adsorption. In this study, as the filtration experiments were carried out at neutral pH, it is not possible that the estrone molecules will become negatively-charged. Hence, the
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X. Jin et al. / Desalination 214 (2007) 83–90 Table 2 Mass balance of estrone in dead-end filtration tests Membrane
Amount of estrone in feedwater (ng)
Amount of estrone in permeate (ng)
Amount of estrone in retenate (ng)
Amount of estrone adsorbed in membrane (ng)
PPO 2 wt%C60 -PPO 10 wt%C60 -PPO
750 750 750
14 6 16
248 162 208
488 582 526
mechanism of charge repulsion is not valid. In this case, we can only attribute these excellent removal performances of the membranes to their adsorption capabilities and steric hindrance effect. To validate this assumption, Table 2 shows the mass balance of estrone in dead-end filtration tests. During the filtration tests, most of estrone molecules were found adsorbing on the three kinds of membranes with different compositions of fullerene. There were 65.1% of estrone molecules adsorbing on pure PPO membrane, 77.6% of estrone molecules adsorbing on 2 wt%C60-PPO membrane and 70.1% of estrone molecules adsorbing on 10 wt%C60-PPO membrane. As estrone removals by all membranes were higher than 96%, it suggested that membrane adsorption during filtration should be the dominating reason for the good removal and steric hindrance also played important role in estrone removal. In some cases removal of trace organics depending on adsorption could be a temporary effect in earlier filtration stage [1]. When the adsorption sites on membrane surface are saturated and accumulated by large amounts of contaminants, there might be a risk of permeate contamination due to desorption of contaminants from membrane. To investigate the adsorption and subsequent retention of estrone in later filtration stage, experiment was conducted to collect 600 ml of permeate instead of the usual 100 ml that was made possible with the use of a reservoir. 10 wt%C60-PPO membrane, with its excellent removal performance and fastest
Fig. 3. Estrone removal by 10 wt%C60 - PPO membrane as a function of permeate volume
permeate flux, was chosen to undergo this longterm filtration. Fig. 3 shows the removal of estrone from aqueous solution by 10 wt%C60PPO membrane as a function of permeate volume. It can be seen that the 10 wt%C60-PPO membrane had a removal of at least 95% in all of the six permeates collected. As mentioned above, the mechanisms attributed to the good removal are most probably the steric hindrance and adsorption capacity of membrane. From mass balance of estrone, it was found that there were 77.3% of estrone molecules adsorbing on the membrane. This value was higher than the one obtained in static adsorption test. The reason for the increase in adsorption capacity is most probably due to the applied pressure in dead-end filtration test. Under the influence of applied pressure, estrone molecules could be pushed to the inside of membrane and then further adsorbed. As a result, this created the increased estrone adsorption in membrane.
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4. Conclusions In this study, the adsorption and removal of natural hormone estrone by PPO membrane with different composition of fullerene were investigated. The addition of fullerene had no significant effect on ultimate adsorption capacities of membranes in static adsorption tests, while the 2 wt%C60-PPO membrane had the fastest adsorption rate, followed by the 10 wt%C60 + PPO and PPO membranes. In dead-end filtration tests, all the three kinds of membrane exhibited very good removal while permeate flux through 10 wt%C60PPO membrane was much higher than that through pure PPO membrane. Results for longterm filtration through 10wt%C60-PPO membrane showed that the membrane was still able to maintain good removal performance of at least 95% after filtration of 600 ml permeate.
[4] [5]
[6]
[7]
[8]
[9]
References [1] L.D. Nghiem, A.I. Schäfer and T.D. Waite, Adsorption interactions between membranes and trace contaminants, Desalination, 147 (2002) 269–274. [2] A.C. Johnson and J.P. Sumpter, Removal of endocrine-disrupting chemicals in activated sludge treatment works, Envir. Sci. Technol., 35 (2001) 4697–4703. [3] T.A. Ternes, M. Stumpf, J. Mueller, K. Haberer, R.D. Wilken and M. Servos, Behavior and occur-
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rence of estrogens in municipal sewage treatment plants — I. Investigations in Germany, Canada and Brazil, Sci. Total Envir., 35 (1999) 81–90. Z. Tim, In vitro bioassays for assessing estrogenic substances, Envir. Sci. Technol., 31 (1997) 613–623. Y. Kiso, Y. Nishimura, T. Kitao and K. Nishimura, Rejection properties of non-phenylic pesticides with nanofiltration membranes, J. Membr. Sci., 171 (2000) 229–237. Y. Kiso, T. Kon, T. Kitao and K. Nishimura, Rejection properties of alkyl phthalates with nanofiltration membranes, J. Membr. Sci., 182 (2001) 205–214. G.A. Polotskaya, D.V. Andreeva and G.K. El’yashevich, Investigation of gas diffusion through films of fullerene-containing poly (phenyleneoxide), Techn. Phys. Lett., 25 (1999) 555–557. G.A. Polotskaya, S.V. Gladchenko and V.N. Zgonnik, Gas diffusion and dielectric studies of polystyrene-fullerene compositions, J. Appl. Polymer. Sci., 85 (2002) 2946–2951. V.I. Berezkin, I.V. Viktorovskii, A.Y. Vul, L.V. Golubev, V.N. Petrova and L.O. Khoroshko, Fullerene single crystals as absorbents of organic compounds, Semiconductors, 37 (2003) 802–810. A.I. Schäfer, L.D. Nghiem and T.D. Waite, Removal of the natural hormone estrone from aqueous solutions using nanofiltration and reverse osmosis, Envir. Sci. Technol., 37 (2003) 182–188. G.A. Polotskaya, Yu.F. Biryulin and V.V. Rozanov, Asymmetric membranes with porous skin based on fullerene-containing polyphenylene oxide, Fullerenes, Nanotubes, Carbon Nanostructures, 12 (2004) 371–376.
Estrogenic compounds removal by fullerene-containing membranes X. Jina, J.Y. Hua*, M.L. Tinta, S.L. Onga, Y. Biryulinb, G. Polotskayac a
Center for Water Research, Division of Environmental Science and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Tel. +65 65164540; Fax: +65 67744202; email: [email protected] b Ioffe Physico-Technical Institute, Russian Academy of Sciences, Polytekhnicheskaya 26, St. Petersburg 194021, Russia c Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy 31, St. Petersburg 199004, Russia Received 24 January 2006; Accepted 15 October 2006
Abstract Estrogenic pollutants present in surface water and treated wastewater could exert significant effects on biota exposed in the receiving environment. Amongst them, the impacts of steroid estrogens such as estrone, estradiol and ethinylestradiol are prominent as they have the highest endocrine disrupting potency than other synthetic endocrine disrupting chemicals (EDCs), despite of low concentration. Since these steroid estrogens are hydrophobic compounds of low volatility, adsorption plays an important role in their removal. Therefore, hydrophobic polymer membranes and strong adsorbent could be effective in removal of estrogens from aqueous phase. In this study, new polymer membranes based on hydrophobic polymer – poly (2, 6-dimethyl-1, 4-phenylene oxide) (PPO) modified by fullerene C60 were studied on removal and adsorptive behaviors of estrogenic compounds. The removal, adsorption rate and capacity of estrone by membranes with different fullerene compositions (PPO; 2%wtC60-PPO; 10%wtC60-PPO) through dead-end filtration and static adsorption experiments were investigated. SEM images demonstrated increase of pore size and porosity in top layer of fullerene-containing membranes compared to pure PPO membrane. Moreover, cross-section of PPO membrane had sponge–like structure while membranes containing C60 (2% and 10%) had finger-like structure. As a result, the permeate flux increased with the increased percentage of fullerene in membrane. However the removal of estrone by PPO membrane with 10% fullerene did not decrease obviously compared with the one without fullerene although the flux increased by about eight times. It was observed that all of the three kinds of membrane showed very good removal of estrone (more than 96%). Results for long-term filtration showed that the 10%wtC60-PPO membrane was able to maintain its excellent removal performance of at least 95%. The results from the static adsorption tests showed that after 12 h, all membranes reached ultimate adsorption of estrone with a similar value of adsorption capacity, although PPO membrane with 2% C60 had the fastest rate of adsorption, followed by PPO membrane with 10% C60 and PPO membrane before reaching the plateau value. This can probably be explained by their pore size on membrane surface and internal structure. *Corresponding author. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.10.019
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Keywords: Estrogens; Adsorption; Fullerene; Membrane; Water reuse, Purification
1. Introduction Water reclamation is an alternative way to augment drinking water resource. However, new emerging contaminants in source water have been heightening the public health concerns. Estrogenic compounds, which mimic or partially mimic the sex steroid hormone estrogens, are the endocrine disruptors mainly concerned in water reclamation. Amongst many kinds of estrogenic compounds, the impact of steroid estrogens such as estrone, estradiol and ethinylestradiol are prominent due to their much higher endocrine disrupting potency than others [1–3]. Estrogenic pollutants present in seawater, surface water and treated wastewater could exert significant effects on biota exposed in the receiving environment. Due to many relative evidences have been observed, estrogenic compounds are believed to affect human reproductive system via food and drinking water. These effects include the development of hormone-dependent cancers, disorders of the reproductive tract, decreased levels of sperm production and compromised reproductive fitness [4]. Because of very low concentration in the range of 1–30 ng/L, the ability of conventional wastewater treatment to remove trace level of estrogenic compounds is limited. Hence, advance technological treatment is essential for more stringent requirements for drinking water and water reclamation. Membrane technology with high selectivity have been more and more frequently applied in water treatment processes due to its operational simplicity, modular and compact equipment comparing to physical and chemical counterparts. In recent years, several researchers have shown that nanofiltration and reverse osmosis are capable of removing trace organic compounds including natural hormones and a wide range of pesticides [5,6]. Due to the
interaction of solution with membrane polymeric matrix, retention can be influenced by both membrane and solution characteristics. Since steroid hormones are hydrophobic organic compounds of low volatility, the mechanism of sorption is expected to play a vital role in their removal during the membrane process. Therefore, membrane material, being hydrophobic in practical usages, which has a relatively high adsorption capacity to achieve a stable and effective removal performance of them, is highly preferred. However, care also must be taken when evaluating the performance of a membrane as desorption may lead to a high risk of bulk release of these trace contaminants. Thus special treatment of membrane is needed for better removal of trace estrogenic compounds. With the rapid development of fullerene science and technology, fullerene-containing polymers have been incorporated into membrane technology since 1992. A series of studies has been shown that polymer membranes modified by fullerene C60 substantially improve their initial properties [7]. However, fullerene-containing polymers were mainly involved in the application of gas separation [7,8]. Moreover, PPO is known as a hydrophobic material having good physicochemical properties. Some results on study of adsorption properties of fullerenes have been presented [9]. Fullerenes as adsorbents of organic compounds in water are much more effective than soot or activated carbon. Adsorption ability of fullerene is realized mainly through dispersion interactions. It has been shown that combination of PPO and С60 molecules gives a donor-acceptor complex. Therefore, it might be a prospect for using fullerene-containing PPO membranes to reject natural steroid estrogens from wastewater. In this study, new asymmetric membranes based on hydrophobic polymer PPO modified by
X. Jin et al. / Desalination 214 (2007) 83–90
fullerene C60 were studied on their ability to adsorb and remove natural hormone estrone. The purposes of this study were (1) to investigate the adsorption of natural hormone estrone on fullerene-containing PPO membranes and (2) to understand the removal performance of fullerenecontaining PPO membranes on estrone removal. 2. Materials and methods 2.1. Membranes Asymmetric polymer membranes based on PPO and its compositions containing 2 and 10%wt C60 were used in this study. The fullerenecontaining compositions containing were obtained by solid-phase interaction of C60 and PPO. Asymmetric membranes were prepared by casting a 6%wt polymer solution containing a PPO or C60-PPO composition in a mixed chloroform: butanol (85:15, %wt) solvent on a glass plate which was immersed in a coagulation bath with ethanol. After 20 h precipitation, membranes were transferred into a bath with 40%wt glycerol in aqueous solution and dried in air. The membranes were washed in distilled water prior to adsorption and filtration tests. The incorporation of fullerene into PPO membrane was accompanied by a color change of the membrane from pure white to brown. 2.2. Membrane removal experiments Membrane removal experiments were conducted by using stirrer cell membrane filtration unit (Millipore, model series 8200, USA) with an 800ml reservoir. 150 ml of sample water with initial estrone concentration of 5 µg/l was placed into the stirrer cell reactor and filtration was conducted at 2 bar pressure. After 100 ml of permeate was collected, filtration was stopped and 50 ml of water remaining in the stirrer cell was collected as retentate. Effective membrane surface area was 28.7 cm2. A magnetic stirrer was used to minimize polarization concentration
85
effects and the stirred speed was fixed at 200 rpm. Instrumental grade nitrogen was used to pressurize the stirred cell. A new membrane was used for each experiment. For long-term filtration, six permeate samples, each of a volume of 100 ml were collected and hence a total of 600 ml of the feed solution was filtered. Parameter used to quantify the membrane removal efficiency was the estrone removal R:
⎛ Cp ⎞ R = 100% × ⎜ 1 − ⎟ ⎝ C0 ⎠
(1)
where Cp was the concentration of estrone in permeate and C0 was the initial concentration of estrone in the stirred cell. 2.3. Static adsorption experiments To compare the adsorption capacity and rate of membrane with different fullerene compositions, static adsorption tests were conducted. Pieces of membrane having total surface area of 20 cm2 were put into 500 ml flask containing 500 ml of sample water with initial estrone concentration of 5 µg/l. The flask was then shaken at 150 rpm to ensure a homogeneous solution. At stipulated timings, 40 ml of sample solution was collected. The concentration of estrone in the sample was monitored to calculate the amount of estrone adsorbed on the membranes. 2.4. Chemicals and solution chemistry Estrone was purchased from Sigma Aldrich (Singapore). Its characteristics have been described elsewhere [1,10]. Diameter of estrone molecule is estimated to be about 0.8 nm using Stokes-Einstein equation. The acid dissociation constant, pKa, of estrone is 10.4. Octanol-water partitioning coefficient (logKow) is 3.43. The background electrolyte of sample water consisted of 1 mM NaHCO3 and 8 mM NaCl and pH was adjusted to 7 with 1N HCl.
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2.5. Estrogen detection
3. Results and discussion
In this study, estrone was extracted from an aqueous sample by solid phase extraction (SPE) cartridge filled with 500mg of C18 (55 µm) from Phenomenex, USA. Before the analytes were extracted, C18 cartridge was conditioned by 7 ml of acetone, 5 ml of methanol and 10 ml of ultrapure water subsequently. The sample water pH was lowered to 3 by using acetic acid before loading through the C18 cartridge at flow rate of 3 ml/min with the aid of a vacuum pump. After sample loading, the cartridge was eluted with 5 ml of acetone. Analytes were re-extracted from the cartridge by passing through another 5 ml of acetone. The final eluant was collected in a brown glass vial and was allowed to dry under a gentle stream of nitrogen. After drying, 0.4 ml of methanol was added into the vial. Then concentrations of estrone in methanol after SPE pretreatment were measured by LC/MS/MS (MDS Sciex API 2000 tandem triple-quadropole mass spectrometer) from Applied Biosystems, USA. Analytes were chromatographed on a 2.1×150 mm column filled with 3.5 µm C18 reversed phase packing (Agilent, USA). High purity nitrogen gas was used as nebulizer, drying, curtain and collision gases. The setting for curtain gas was 40 psi. For estrone, the declustering potential was 30 V. MRM mode was chosen for quantification. The ion spay voltage was !4500 V and the probe temperature was 450EC. All the source and instrument parameters for monitoring estrone were optimized by standard solutions by a syringe pump. After observing collision-induced dissociation (CID) spectra obtained by full-scan production experiments, the following MRM pairs were chosen: 269.2/145.0; 269.2/143.0.
3.1. Analysis of membrane structure
2.6. Membrane structure analysis Asymmetric membrane structure analyses were performed using a scanning electron microscope (SEM), Phillips XL30 FEG SEM, in this study.
Asymmetric membranes structure was showed by scanning electron microscopy. Fig. 1 demonstrates SEM micrographs of skin surface and the cross-section of PPO and 10 wt%C60-PPO membranes. All membranes consisted of a porous support and a dense top layer which also contained pores. The structures of PPO and C60-PPO membranes differed greatly. The micrograph of PPO top layer shows that the skin surface was rather smooth; surface pores were dead-end with thin partitions and similar thin partitions formed a sponge-like porous structure throughout the cross section. As for 10%wt С60-PPO membrane, the skin surface was sufficiently smooth but inhomogeneous. Surface pores were also deadend but deeper. The cross section of 10%wt C60PPO (just as that of 2%wt C60-PPO) membrane demonstrates the finger-like structure. A difference can be seen in the thickness of PPO and C60PPO membranes prepared in the same conditions. Furthermore, pore size and porosity of a dense top layer are higher in the case of C60-PPO. Physical cross-linking of several PPO chains by C60 molecules leads to increasing microheterogeneity of the system in solution and favours the formation of more developed porous structure of fullerene-containing membranes [11]. 3.2. Adsorption of estrone on fullerenecontaining PPO membranes The adsorption percentages of estrone on membrane with different composition of fullerene at static adsorption condition are shown in Fig. 2. The dependence of adsorption capacity on time demonstrates high adsorption activity of both PPO and fullerene-containing PPO membranes with respect to estrone. It was observed that after 12 h, adsorption of estrone on the three kinds of membrane had attained a plateau value and the ultimate adsorption extents by all membranes
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Fig. 1. SEM images of skin surface and cross-section for PPO and 10 wt%C60-PPO membranes.
Fig. 2. Adsorption of estrone on membrane with different composition of fullerene.
were similar. We attempt to supply an explanation of these observed results. The adsorption of estrone molecules on membrane depends on the following facts: size of contact area in the membrane and the presence of C60 molecules that is known as lipophilic agent which can give additional sites in the membrane for estrone adsorption. It is evident that the amount of adsorption sites depends on the contact area. Porous structure of PPO and C60-PPO membranes differs greatly. Different porosity implies different internal area that is available for adsorption. Moreover, the PPO membrane exhibits the largest internal area due to its sponge-like porous structure. On the other hand, membranes containing
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2 wt% and 10 wt% C60 have additional adsorption sites due to the fact that C60 molecules are active adsorbents but their finger-like internal surfaces possess smaller area than that of PPO membrane. As a result of the opposition of these two factors, the three kinds of membrane demonstrated similar adsorption capacity of estrone at equilibrium state. In addition, it was found that PPO membrane with 2 wt% C60 had the fastest rate of adsorption, followed by PPO membrane with 10 wt% C60 and pure PPO membrane before reaching the plateau value. The reason seems to be due to the difference in their pore size on surface and internal structure. For adsorption rate, one of the influencing factors could be diffusion. The larger the pore size, the quicker the diffusion of estrone from the proximity of the membrane surface into membrane internal porous structure. Based on SEM images (Fig. 1), C60-PPO membrane has bigger pore size on the surface than that of the PPO membrane. On the other hand, the PPO membrane showed bigger internal area than that of C60-PPO membrane. Once estrone molecules enter into membrane internal structure, bigger internal area of membrane may lead to better diffusion of estrone inside the membrane. Combining these two opposite factors, an optimum in terms of adsorption rate was realized in 2 wt% C60-PPO membrane. 3.3. Estrone removal by membranes with different compositions of fullerene Table 1 shows the data on flux and estrone removal in filtration experiments with ultrapure water and aqueous solution of estrone by using PPO membranes containing different C60 content at 2 bar. It can be seen clearly that fullerene incorporation improved the pure water flux by 8 times with the increased percentage of fullerene in membrane that indicated modifying PPO with fullerene was potentially useful for improving the transport properties of the membrane. This
Table 1 Flux and estrone removal in filtration tests Membrane
PPO 2 wt%C60 PPO 10 wt%C60 PPO
Pure water flux (×105 cm/s)
Filtration of estrone solution Permeate flux (×105 cm/s)
Estrone removal (%)
33.6 60.6
22.8 39.2
97.2 98.8
273.0
193.6
96.8
phenomenon was not surprising since fullerene incorporation lead to the increases in membrane surface pore size as mentioned earlier. It was also observed that fullerene incorporation improved the permeate flux by around 8 times with increased percentage of fullerene in membrane during filtration of aqueous solution of estrone. It was, however, interesting to note that estrone removal by 10 wt%C60+PPO membrane was similar with the one without fullerene. It was shown in the dead-end filtration tests that all of the three kinds of membrane provided very good removal of estrone (more than 96%). Membranes potentially useful in organic retention should possess high permeability and high removal. However, for most membrane materials these two properties are antipathetic. Based on the results shown above, 10 wt%C60-PPO membrane has an optimum combination of permeability and removal and would have a prospective in practical application for estrone removal comparing with the original PPO membrane. In membrane technology, there are three main mechanisms in which contaminants can be retained. They are namely charge repulsion, steric hindrance and adsorption. In this study, as the filtration experiments were carried out at neutral pH, it is not possible that the estrone molecules will become negatively-charged. Hence, the
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X. Jin et al. / Desalination 214 (2007) 83–90 Table 2 Mass balance of estrone in dead-end filtration tests Membrane
Amount of estrone in feedwater (ng)
Amount of estrone in permeate (ng)
Amount of estrone in retenate (ng)
Amount of estrone adsorbed in membrane (ng)
PPO 2 wt%C60 -PPO 10 wt%C60 -PPO
750 750 750
14 6 16
248 162 208
488 582 526
mechanism of charge repulsion is not valid. In this case, we can only attribute these excellent removal performances of the membranes to their adsorption capabilities and steric hindrance effect. To validate this assumption, Table 2 shows the mass balance of estrone in dead-end filtration tests. During the filtration tests, most of estrone molecules were found adsorbing on the three kinds of membranes with different compositions of fullerene. There were 65.1% of estrone molecules adsorbing on pure PPO membrane, 77.6% of estrone molecules adsorbing on 2 wt%C60-PPO membrane and 70.1% of estrone molecules adsorbing on 10 wt%C60-PPO membrane. As estrone removals by all membranes were higher than 96%, it suggested that membrane adsorption during filtration should be the dominating reason for the good removal and steric hindrance also played important role in estrone removal. In some cases removal of trace organics depending on adsorption could be a temporary effect in earlier filtration stage [1]. When the adsorption sites on membrane surface are saturated and accumulated by large amounts of contaminants, there might be a risk of permeate contamination due to desorption of contaminants from membrane. To investigate the adsorption and subsequent retention of estrone in later filtration stage, experiment was conducted to collect 600 ml of permeate instead of the usual 100 ml that was made possible with the use of a reservoir. 10 wt%C60-PPO membrane, with its excellent removal performance and fastest
Fig. 3. Estrone removal by 10 wt%C60 - PPO membrane as a function of permeate volume
permeate flux, was chosen to undergo this longterm filtration. Fig. 3 shows the removal of estrone from aqueous solution by 10 wt%C60PPO membrane as a function of permeate volume. It can be seen that the 10 wt%C60-PPO membrane had a removal of at least 95% in all of the six permeates collected. As mentioned above, the mechanisms attributed to the good removal are most probably the steric hindrance and adsorption capacity of membrane. From mass balance of estrone, it was found that there were 77.3% of estrone molecules adsorbing on the membrane. This value was higher than the one obtained in static adsorption test. The reason for the increase in adsorption capacity is most probably due to the applied pressure in dead-end filtration test. Under the influence of applied pressure, estrone molecules could be pushed to the inside of membrane and then further adsorbed. As a result, this created the increased estrone adsorption in membrane.
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4. Conclusions In this study, the adsorption and removal of natural hormone estrone by PPO membrane with different composition of fullerene were investigated. The addition of fullerene had no significant effect on ultimate adsorption capacities of membranes in static adsorption tests, while the 2 wt%C60-PPO membrane had the fastest adsorption rate, followed by the 10 wt%C60 + PPO and PPO membranes. In dead-end filtration tests, all the three kinds of membrane exhibited very good removal while permeate flux through 10 wt%C60PPO membrane was much higher than that through pure PPO membrane. Results for longterm filtration through 10wt%C60-PPO membrane showed that the membrane was still able to maintain good removal performance of at least 95% after filtration of 600 ml permeate.
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