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Effect of gamma-ray irradiation at low doses on the performance of PES ultrafiltration membrane
Radiation Physics and Chemistry 127 (2016) 127–132
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Effect of gamma-ray irradiation at low doses on the performance of PES ultrafiltration membrane Xue Zhang a,n, Lixia Niu a,b, Fuzhi Li a, Suping Yu a, Xuan Zhao a,n, Hongying Hu b,c a Collaborative Innovation Center for Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China b State Environmental Protection Key Laboratory of Microorganism Application and Risk Control (MARC), Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China c Environmental Simulation and Pollution Control State Key Joint Laboratory, School of Environment, Tsinghua University, Beijing 100084, PR China
H I G H L I G H T S
The fouling properties of PES UF membranes changed after low dose irradiation. Membrane surface was hydrophilic modified at even 10 kGy irradiation. The flux decreased more at higher dose irradiation due to more pore blocking. The depositions on the 75 kGy irradiated membrane were less than the others.
art ic l e i nf o
a b s t r a c t
Article history: Received 3 June 2016 Received in revised form 20 June 2016 Accepted 28 June 2016 Available online 28 June 2016
The influence of gamma irradiation on the performance of polyether sulfone (PES) ultrafiltration (UF) membrane was investigated at low absorbed doses (0–75 kGy) using a cobalt source. The performance of the UF membranes was tested using low level radioactive wastewater (LLRW) containing three types of surfactants (anionic, cationic and nonionic surfactants). The physical and chemical properties of membrane surface were analyzed, and relationships between these properties and separation performance and fouling characteristics were determined. At 10–75 kGy irradiation, there were no significant changes observed in the membrane surface roughness or polymer functional groups, however the contact angle decreased sharply from 92° to ca. 70° at irradiation levels as low as 10 kGy. When membranes were exposed to the surfactant-containing LLRW, the flux decreased more sharply for higher dosed irradiated membranes, while flux in virgin membranes increased during the filtration processes. The study highlights that fouling properties of membrane may be changed due to the changes of surface hydrophilicity at low dose irradiation, while other surface properties and retentions remain stable. Therefore, a membrane fouling test with real or simulated wastewater is recommended to fully evaluate the membrane irradiation resistance. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Membrane fouling Gamma irradiation PES membrane
1. Introduction A significant amount of low level radioactive wastewater (LLRW) is generated from the nuclear power industry and wastewater treatment is required to meet stringent environmental requirements on radionuclides. Conventional treatment methods (e.g., evaporation and ion exchange) are often associated with high energy consumption and the production of large amounts of n
Corresponding authors. E-mail addresses: [email protected] (X. Zhang), [email protected] (X. Zhao). http://dx.doi.org/10.1016/j.radphyschem.2016.06.030 0969-806X/& 2016 Elsevier Ltd. All rights reserved.
radioactive solid wastes. Membrane processes exhibit several advantages over conventional processes, such as low energy consumption, easy operation, and environmental friendly, and have seen increased application in radioactive wastewater treatment (Ambashta and Sillanpää, 2012; Zakrzewska-Trznadel et al., 2001). For example, reverse osmosis (RO), capable of rejecting most ions, was a key wastewater treatment process used following the Fukushima Daiichi nuclear power plant accident (Rana et al., 2013). Ultrafiltration (UF), which can better remove colloidal substance and suspended particles, is always combined with other technologies, such as coagulation to remove nuclides (ZakrzewskaTrznadel, 2003).
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X. Zhang et al. / Radiation Physics and Chemistry 127 (2016) 127–132
Membranes are exposed to ionizing radiation during the treatment of LLRW. Most membranes are manufactured from polymers that readily undergo degradation when exposed to gamma rays (Placek et al., 2008). Therefore, it is necessary to determine the influence of irradiation on the durability and performance of polymeric membranes (Chmielewski and Harasimowicz, 1992). The physical and chemical properties of most polymeric membranes have been shown to remain stable up to doses of 100 kGy (IAEA, 2004). For example, UF membranes composed of polysulfone (PS) and polyamide (PA) were found to be resistant to gamma and electron radiation, however, the permeate flux and the retention coefficient of dextrine by PS membrane showed a sharp decrease when the doses exceeded 100 kGy (Chmielewski and Harasimowicz, 1992). Gamma radiation of less than 20 Gy has been shown to have minimal effect on both the structure and transport parameters of the aromatic polyamide composite RO membranes (Arnal et al., 2003). For cross-linked aromatic polyamide RO membranes, the salt retention remained above 88% until irradiation reached 200 kGy, and the membrane maintained ca. 90% of its original water flux (Nakase et al., 1994). The permeability of PA RO membranes irradiated at 200 kGy was found to decrease slightly, however there was not associated change in retention (Combernoux et al., 2016). In LLRW treatment systems, the UF membranes usually act as the first stage of purification in multistage treatment, and thus, the UF membranes generally absorb considerably higher doses than the subsequent RO membranes. The previous studies have shown that the membrane surface properties and retentions remain stable under low dose irradiation (o100 kGy). However, there is little knowledge about the effects of irradiation on membrane fouling properties. Membrane fouling can adversely impact the effectiveness of LLRW treatments. Membrane fouling leads to an increase in radioactive solid waste, and more frequent membrane cleaning, resulting in a reduction in membrane life and an increase in radioactive solid waste. Therefore, control of membrane fouling plays a key role in the stable operation of a membrane system. For LLRW produced from nuclear power operations, surfactants compose the main organic pollutants (Chen et al., 2016), and have been shown to cause membrane fouling through adsorption onto the membrane surface and pores (Kaya et al., 2006; Urbanski et al., 2002). To facilitate the application of membrane technology in nuclear power plants, it is necessary to investigate the effects of irradiation on the membrane fouling caused by surfactants. In this study, the effects of gamma irradiation on the properties of polyether sulfone (PES) UF membranes were studied at low absorbed doses (o 100 kGy) produced by a cobalt source. Membrane performance was evaluated by ultrafiltration of LLRW containing three types (anionic, cationic, and nonionic) of surfactants. The physical and chemical properties of membrane surface (e.g., surface roughness, functional groups, contact angle) were analyzed, and linked to the separation performance and fouling characteristics. Results from this study can help guide the future design and operation of membrane technologies for LLRW treatment.
2. Experimental 2.1. Materials and chemicals All chemicals used in the study were of analytical grade. Four salts were obtained from Beijing Chemical Works (Beijing, China): cesium nitrate (CsNO3), cobalt nitrate hexahydrate (Co (NO3)2 6H2O), strontium nitrate (Sr(NO3)2), and silver nitrate (AgNO3). Feed solutions were prepared by dissolving required amounts of salts in highly demineralized water
(conductivity o1 μs/cm, pH ¼7.4), with nuclide concentrations of 1 mg/L. The anionic surfactant sodium dodecyl benzene sulfonate (SDBS), nonionic surfactants Tween-80 and cationic surfactant cetyltrimethyl ammonium bromide (CTAB) were obtained from Its Group Chemical Reagent Co., Ltd. (Beijing, China). The concentration of surfactants in all feed solutions was 200 mg/L. Flat PES UF membranes (SEPRO Membranes, Inc., USA) with a molecular weight cut-off (MWCO) of 5 kDa were used in experiments. The membranes were soaked in deionized water for 24 h before irradiation or filtration experiments. 2.2. Irradiation condition The membrane samples were immerged in ultrapure water in a sealed 50 mL glass tube. Gamma irradiation was applied using a 60 Co source at room temperature (ca. 250 °C) at a constant dose rate of 0.24 kGy/min. Four gamma irradiation levels were tested: 10 kGy, 25 kGy, 50 kGy and 75 kGy. The cumulated energy absorbed in the sample was measured by dosimeters with a known response to irradiation, proportional to the exposure time (Traboulsi et al., 2013). Irradiation time ranged from 41 to 312 min based on the total absorbed dose. The samples immerged in ultrapure water were stored in a refrigerator at 4 °C after irradiation. 2.3. Filtration setup A dead-end UF filtration cell connected to a 4 L feed reservoir was used for experiments as described previously (Niu et al., 2015). Feed solution from the reservoir was transferred into the UF cell using nitrogen gas at a pressure of 0.1 MPa. The filtration cell (Amicon 8200, Millipore, USA) had an effective membrane filtration area of 28.7 cm2, and a volume of 200 mL. Experiments were conducted at room temperature (25 °C) and at a stirring speed of 120 rpm. The permeate flux was measured using a balance connected to a computer. After each filtration cycle, 200 mL concentrate was generated. The water flux of the membrane was determined by filtering deionized water through the membrane until a stable permeate flux was obtained. Feed water was added to the reservoir and filtered through the membrane for 12 h. To determine the concentrations of nuclides and surfactants, filtrate of the first hour, the final concentrate, and the feed water were sampled during each experiment. 2.4. Analytical methods Concentrations of the metal ions Sr(II), Co(II), Cs(I), and Ag (I) were determined using inductively coupled plasma mass spectrometry (iCAP Q, ThermoFisher Scientific, Waltham, MA USA). The concentrations of surfactants in the solutions were determined using total organic carbon (TOC), which was measured using a Shimadzu Total Organic Carbon Analyzer (Shimadzu Corporation, Japan). Surface morphologies of the raw and fouled membranes were observed using a scanning electron microscope coupled with an energy dispersive spectrometer (SEM/EDS, Sirion 200, FEI Inc., USA). The membrane samples were freeze-dried and coated with a thin layer of gold prior to analysis. The functional groups on the membrane surfaces were detected using attenuated total reflectance - Fourier Transform Infrared Spectrometer (ATR-FTIR, Spectrum One, PerkinElmer Inc., USA). The surface roughness of the freeze-dried membrane was determined by Atomic Force Microscope (AFM, SPA-300HV, SEIKO, Japan) in tapping mode and images were processed using a NanoNavi Station (Version 5.00, SII Nano Technology Inc., Japan). The parameter Ra represents the average of the mean roughness (arithmetic average of the deviation from the center plane) of different spots and scanning sizes,
X. Zhang et al. / Radiation Physics and Chemistry 127 (2016) 127–132
and P-V represented the height difference between a peak and a valley. The contact angle of the membrane was measured using a contact angle analyzer (DSA100, Hamburg Germany).
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Table 1 Surface parameter and pure water flux of membrane before and after irradiation. Absorbed dose (kGy)
Ra (nm) P-V (nm) Contact angle (°)
Pure water flux (L/ (m2 h))
0 10 25 50 75
4.2 3.8 3.8 4.8 4.3
23.8 28.7 29.2 31.0 32.4
2.5. Data analysis The permeate flux (Ji, L/m2/h) was calculated as
Ji = 60 ×
mi ρ×A
(1)
where mi is the effluent mass per minute (kg/min), ρ is solution density (kg/L), and A is the outside surface area of the membrane (m2). The relative flux (RF) was calculated as
RF =
Ji J0
(2)
where J0 is pure water flux. The retention of nuclides and surfactants was calculated as
R=
cR − cP × 100% cR
(3)
where CP and CR are the concentration of pollutants in the permeate and feed solutions, respectively (μg/L for nuclides and mg/L for surfactants).
3. Results and discussion
62 124 114 128 102
92 70 70 72 68
attributed to a loss in hydrogen bonds (Sengupta et al., 2005). The FTIR data indicated that radical reactions occurring in the membrane and the network structure may have been relaxed due to a loss of hydrogen inter-chains bonds (Combernoux et al., 2015). The surface roughness was measured using AFM (Supporting information, Fig. S1) and the roughness parameters are shown in Table 1. The Ra values ranged from 3.8 nm to 4.8 nm, while the P-V values increased from 62 nm to 128 nm. No significant changes in membrane surface roughness were observed after irradiation. However, the membrane was observed to become more hydrophilic following irradiation. The contact angle of virgin membrane was 92°, compared with 68–72° after irradiation (Table 1). In accordance with these results, the pure water fluxes increased from 23.8 L/(m2 h) for the virgin membrane to 32.4 L/(m2 h) with 75 kGy-irradiated membrane. The flux was found to increase linearly with the absorbed dose. These changes are consistent with the loss in hydrogen bonds observed in the FTIR data.
3.1. Effects of irradiation on membrane surface The virgin and irradiated membrane samples were characterized using several analytical methods. The functional groups on the membrane surfaces were identified in ATR-FTIR spectra (Fig. 1). Several major peaks were observed in the spectra of virgin PES membrane, including bands at 1582 cm 1, 1491 cm 1 and 1409 cm 1, which correspond to benzene skeleton vibration, a band at 3107 cm 1 corresponding to C–H stretching in the benzene skeleton, bands at 1322 cm 1 and 1245 cm 1 corresponding to C–O–C stretching, and bands at 1151 cm 1 and 1105 cm 1 corresponding to S ¼O stretching (Tang et al., 2010). No significant changes in these major peaks were observed among the membrane samples after irradiation with different doses. However, the adsorption values at 3396 cm 1, which correspond to O-H stretching decreased after irradiation. This decrease may be
Fig. 1. ATR-FTIR spectra of membrane samples after irradiation of various doses.
3.2. Effects of irradiation on membrane performance during LLRW treatment 3.2.1. Flux variation The influence of absorbed dose on membrane performance was evaluated using surfactant-containing LLRW. The flux variations were observed to be significantly different for the irradiated membranes (Fig. 2) compared with the virgin membrane. For example, when membranes were exposed to SDBS solutions, the initial flux of irradiated membranes (ca. 60%) was considerably lower than that of the virgin membrane (73%). The normalized flux of the virgin and 10 kGy-irradiated membrane followed similar trends, with decreases of 13–20% observed during the first 200 min after which fluxes increased by 30–35% over the following 400 min. The flux variations for membranes irradiated at doses of 25–75 kGy were significantly smaller, with fluxes decreasing by 10–20% during the first 200 min and then remaining at a steady value or decreasing in the following 400 min. The initial decrease in flux may be attributed to pore blocking by surfactant monomers, while the subsequent increase may be the result of hydrophilic modification of the membrane surface by surfactant micelles. Therefore, the irradiated membranes exhibited a lower affinity for the surfactant micelles compared with the virgin membrane, while the monomer adsorption properties of pores were similar among virgin and irradiated membranes. Similar phenomena were observed with CTAB and Tween-80 solutions. The flux was found to increase for the virgin membranes, while flux decreased with irradiated membranes. At higher the absorbed doses, the flux decreased more rapidly, which may indicated a higher susceptibility of irradiated membranes to pore blocking by surfactants. The more hydrophilic surface of the irradiated membranes was less susceptible to adsorption of surfactants, while the membrane pores retained a high affinity for monomer surfactants. In summary, these results show that irradiated membranes were more easily fouled due to pore blocking.
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Fig. 3. Retention of pollutants under different conditions. Fig. 2. Flux variation of virgin and irradiated membrane.
3.2.2. Retention of pollutants The retentions of surfactants and nuclides are shown in Fig. 3. For SDBS solutions, the retentions by the virgin membrane were 70%, 97%, 97%, 91%, 60% for SDBS, Co(II), Sr(II), Ag(I) and Cs(I), respectively. For CTAB solutions, the retentions by the virgin membrane were 59%, 96%, 95%, 97%, 28% for CTAB, Co(II), Sr(II), Ag (I) and Cs(I), respectively. For Tween-80 solutions, the retentions by the virgin membrane were 70%, 29%, 32%, 28%, 27% for Tween80, Co(II), Sr(II), Ag(I) and Cs(I), respectively. The retentions of nuclides were considerably lower in nonionic surfactant solutions compared with anionic and cationic surfactant solutions. The retentions of surfactants and nuclides exhibited fluctuations of less than 10% for membranes irradiated at different doses. However, no positive or negative correlations between retention and absorbed
dose was identified. These results indicate that absorbed doses below 75 kGy had minimal effect on membrane pore sizes, since size exclusion is the key factor for UF membranes. The results are consistent with previous reports that the retention of reverse osmosis membranes remains stable at doses lower than 100 kGy (Combernoux et al., 2015). 3.2.3. Membrane fouling analysis The membrane samples fouled with SDBS solutions were analyzed using SEM/EDS (Fig. 4). Images show that deposition on the 75 kGy irradiated membrane was significantly lower than that on other membrane samples. The nuclides deposited on the membranes were not detectable by EDS for all membrane samples (Supporting information, Fig. S2). Similar results were observed for CTAB and Tween-80 samples (Supporting information, Fig. S3 and S4). The lower deposition on 75 kGy-irradiated membrane may be
X. Zhang et al. / Radiation Physics and Chemistry 127 (2016) 127–132
131
Fig. 4. SEM graphs of virgin and irradiated membranes fouled by SDBS solutions.
attributed to the hydrophilic modification of membrane surface by irradiation and the lower adsorption capacity of the more hydrophilic membranes (Boussu et al., 2007). The limited deposition of surfactants on irradiated membrane surface resulted in less surface modification, as well as minimal increase in flux compared with virgin membranes.
Technology and Industry for National Defense, Changjiang Scholars and Innovative Research Team in University [IRT-13026]; and the Science & Technology Project of Tsinghua University [Grant No. 2014z21021].
Appendix A. Supporting information 4. Conclusions The resistance of PES UF membranes to irradiation by gamma rays was investigated at 0–75 kGy using a 60Co source. AFM and FTIR data showed that there were no significant changes to the membrane surfaces within the tested dose range. However, the contact angle decreased sharply from 92° to ca. 70° even at doses as low as 10 kGy. Hydrophilic modification of the membrane following irradiation may be attributed to the loss of hydrogen bond due to radical reactions at the membrane surface. In accordance with these results, the pure water flux increased linearly with the absorbed dose. The flux of the virgin membranes increased during treatment of surfactant-containing LLRW, while the flux for irradiated membranes decreased. The flux decreased more sharply in membranes exposed to higher doses. Results from SEM analysis showed that deposition on the 75 kGy-irradiated membrane was the lowest among all tested conditions, which may be attributed to higher hydrophilicity after irradiation and lower affinity for surfactant micelles. However, pore blocking by the monomers was enhanced following irradiation, leading to a decrease in flux. The retentions of nuclides and surfactants were similar for both virgin and irradiated membranes. The results show that the fouling properties of PES membrane are susceptible to gamma irradiation compared with retentions, and need to be paid more attentions. Therefore, a membrane fouling test with real or simulated wastewater is recommended to fully evaluate the membrane irradiation resistance.
Acknowledgements This work was supported by State administration of Science,
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.radphyschem. 2016.06.030.
References Ambashta, R.D., Sillanpää, M.E.T., 2012. Membrane purification in radioactive waste management: a short review. J. Environ. Radioact. 105, 76–84. Arnal, J.M., Sancho, M., Verdú, G., Campayo, J.M., Villaescusa, J.I., 2003. Treatment of 137Cs liquid wastes by reverse osmosis Part I. Preliminary tests. Desalination 154 (1), 27–33. Boussu, K., Kindts, C., Vandecasteele, C., Van der Bruggen, B., 2007. Surfactant fouling of nanofiltration membranes: measurements and mechanisms. Chemphyschem 8 (12), 1836–1845. Chen, D., Zhao, X., Li, F., Zhang, X., 2016. Influence of surfactant fouling on rejection of trace nuclides and boron by reverse osmosis. Desalination 377, 47–53. Chmielewski, A.G., Harasimowicz, M., 1992. Influence of gamma and electron irradiation on transport properties of ultrafiltration membrane. Nukleonika 37, 61–70. Combernoux, N., Labed, V., Schrive, L., Wyart, Y., Carretier, E., Moulin, P., 2016. Effect of gamma irradiation at intermediate doses on the performance of reverse osmosis membranes. Radiat. Phys. Chem. 124, 241–245. Combernoux, N., Schrive, L., Labed, V., Wyart, Y., Carretier, E., Benony-Rhodier, A., Moulin, P., 2015. Study of polyamide composite reverse osmosis membrane degradation in water under gamma rays. J. Membr. Sci. 480, 64–73. IAEA, 2004. Application of Membrane Technologies for Liquid Radioactive Waste Processing, Vienna. Kaya, Y., Aydiner, C., Barlas, H., Keskinler, B., 2006. Nanofiltration of single and mixture solutions containing anionics and nonionic surfactants below their critical micelle concentrations (CMCs). J. Membr. Sci. 282 (1–2), 401–412. Nakase, Y., Yanagi, T., Uemura, T., 1994. Irradiation effects on properties of reverseosmosis membrane-based on cross-linked aromatic polyamide. J. Nucl. Sci. Technol. 31 (11), 1214–1221. Niu, L.X., Zhang, X., Zhao, X., Hu, H.Y., 2015. EDTA fouling in dead-end ultrafiltration of low level radioactive wastewater. Nucl. Eng. Des. 295, 276–282. Placek, V., Hnat, V., Pejsa, R., Kohout, T., 2008. Testing of polysulfone for applications in nuclear facilities. J. Appl. Polym. Sci. 109 (4), 2395–2399. Rana, D., Matsuura, T., Kassim, M.A., Ismail, A.F., 2013. Radioactive decontamination of water by membrane processes—a review. Desalination 321, 77–92.
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Sengupta, R., Tikku, V.K., Somani, A.K., Chaki, T.K., Bhowmick, A.K., 2005. Electron beam irradiated polyamide-6,6 films—I: characterization by wide angle X-ray scattering and infrared spectroscopy. Radiat. Phys. Chem. 72 (5), 625–633. Tang, H.W., Zhang, L.P., Li, S., Zhao, G.J., Qin, Z., Sun, S.Q., 2010. Study on spectroscopic characterization and property of pes/micro-nano cellulose composite membrane material. Spectrosc. Spectr. Anal. 03, 630–634. Traboulsi, A., Labed, V., Dauvois, V., Dupuy, N., Rebufa, C., 2013. Gamma radiation effect on gas production in anion exchange resins. Nucl. Instrum. Methods Phys.
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Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Effect of gamma-ray irradiation at low doses on the performance of PES ultrafiltration membrane Xue Zhang a,n, Lixia Niu a,b, Fuzhi Li a, Suping Yu a, Xuan Zhao a,n, Hongying Hu b,c a Collaborative Innovation Center for Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China b State Environmental Protection Key Laboratory of Microorganism Application and Risk Control (MARC), Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China c Environmental Simulation and Pollution Control State Key Joint Laboratory, School of Environment, Tsinghua University, Beijing 100084, PR China
H I G H L I G H T S
The fouling properties of PES UF membranes changed after low dose irradiation. Membrane surface was hydrophilic modified at even 10 kGy irradiation. The flux decreased more at higher dose irradiation due to more pore blocking. The depositions on the 75 kGy irradiated membrane were less than the others.
art ic l e i nf o
a b s t r a c t
Article history: Received 3 June 2016 Received in revised form 20 June 2016 Accepted 28 June 2016 Available online 28 June 2016
The influence of gamma irradiation on the performance of polyether sulfone (PES) ultrafiltration (UF) membrane was investigated at low absorbed doses (0–75 kGy) using a cobalt source. The performance of the UF membranes was tested using low level radioactive wastewater (LLRW) containing three types of surfactants (anionic, cationic and nonionic surfactants). The physical and chemical properties of membrane surface were analyzed, and relationships between these properties and separation performance and fouling characteristics were determined. At 10–75 kGy irradiation, there were no significant changes observed in the membrane surface roughness or polymer functional groups, however the contact angle decreased sharply from 92° to ca. 70° at irradiation levels as low as 10 kGy. When membranes were exposed to the surfactant-containing LLRW, the flux decreased more sharply for higher dosed irradiated membranes, while flux in virgin membranes increased during the filtration processes. The study highlights that fouling properties of membrane may be changed due to the changes of surface hydrophilicity at low dose irradiation, while other surface properties and retentions remain stable. Therefore, a membrane fouling test with real or simulated wastewater is recommended to fully evaluate the membrane irradiation resistance. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Membrane fouling Gamma irradiation PES membrane
1. Introduction A significant amount of low level radioactive wastewater (LLRW) is generated from the nuclear power industry and wastewater treatment is required to meet stringent environmental requirements on radionuclides. Conventional treatment methods (e.g., evaporation and ion exchange) are often associated with high energy consumption and the production of large amounts of n
Corresponding authors. E-mail addresses: [email protected] (X. Zhang), [email protected] (X. Zhao). http://dx.doi.org/10.1016/j.radphyschem.2016.06.030 0969-806X/& 2016 Elsevier Ltd. All rights reserved.
radioactive solid wastes. Membrane processes exhibit several advantages over conventional processes, such as low energy consumption, easy operation, and environmental friendly, and have seen increased application in radioactive wastewater treatment (Ambashta and Sillanpää, 2012; Zakrzewska-Trznadel et al., 2001). For example, reverse osmosis (RO), capable of rejecting most ions, was a key wastewater treatment process used following the Fukushima Daiichi nuclear power plant accident (Rana et al., 2013). Ultrafiltration (UF), which can better remove colloidal substance and suspended particles, is always combined with other technologies, such as coagulation to remove nuclides (ZakrzewskaTrznadel, 2003).
128
X. Zhang et al. / Radiation Physics and Chemistry 127 (2016) 127–132
Membranes are exposed to ionizing radiation during the treatment of LLRW. Most membranes are manufactured from polymers that readily undergo degradation when exposed to gamma rays (Placek et al., 2008). Therefore, it is necessary to determine the influence of irradiation on the durability and performance of polymeric membranes (Chmielewski and Harasimowicz, 1992). The physical and chemical properties of most polymeric membranes have been shown to remain stable up to doses of 100 kGy (IAEA, 2004). For example, UF membranes composed of polysulfone (PS) and polyamide (PA) were found to be resistant to gamma and electron radiation, however, the permeate flux and the retention coefficient of dextrine by PS membrane showed a sharp decrease when the doses exceeded 100 kGy (Chmielewski and Harasimowicz, 1992). Gamma radiation of less than 20 Gy has been shown to have minimal effect on both the structure and transport parameters of the aromatic polyamide composite RO membranes (Arnal et al., 2003). For cross-linked aromatic polyamide RO membranes, the salt retention remained above 88% until irradiation reached 200 kGy, and the membrane maintained ca. 90% of its original water flux (Nakase et al., 1994). The permeability of PA RO membranes irradiated at 200 kGy was found to decrease slightly, however there was not associated change in retention (Combernoux et al., 2016). In LLRW treatment systems, the UF membranes usually act as the first stage of purification in multistage treatment, and thus, the UF membranes generally absorb considerably higher doses than the subsequent RO membranes. The previous studies have shown that the membrane surface properties and retentions remain stable under low dose irradiation (o100 kGy). However, there is little knowledge about the effects of irradiation on membrane fouling properties. Membrane fouling can adversely impact the effectiveness of LLRW treatments. Membrane fouling leads to an increase in radioactive solid waste, and more frequent membrane cleaning, resulting in a reduction in membrane life and an increase in radioactive solid waste. Therefore, control of membrane fouling plays a key role in the stable operation of a membrane system. For LLRW produced from nuclear power operations, surfactants compose the main organic pollutants (Chen et al., 2016), and have been shown to cause membrane fouling through adsorption onto the membrane surface and pores (Kaya et al., 2006; Urbanski et al., 2002). To facilitate the application of membrane technology in nuclear power plants, it is necessary to investigate the effects of irradiation on the membrane fouling caused by surfactants. In this study, the effects of gamma irradiation on the properties of polyether sulfone (PES) UF membranes were studied at low absorbed doses (o 100 kGy) produced by a cobalt source. Membrane performance was evaluated by ultrafiltration of LLRW containing three types (anionic, cationic, and nonionic) of surfactants. The physical and chemical properties of membrane surface (e.g., surface roughness, functional groups, contact angle) were analyzed, and linked to the separation performance and fouling characteristics. Results from this study can help guide the future design and operation of membrane technologies for LLRW treatment.
2. Experimental 2.1. Materials and chemicals All chemicals used in the study were of analytical grade. Four salts were obtained from Beijing Chemical Works (Beijing, China): cesium nitrate (CsNO3), cobalt nitrate hexahydrate (Co (NO3)2 6H2O), strontium nitrate (Sr(NO3)2), and silver nitrate (AgNO3). Feed solutions were prepared by dissolving required amounts of salts in highly demineralized water
(conductivity o1 μs/cm, pH ¼7.4), with nuclide concentrations of 1 mg/L. The anionic surfactant sodium dodecyl benzene sulfonate (SDBS), nonionic surfactants Tween-80 and cationic surfactant cetyltrimethyl ammonium bromide (CTAB) were obtained from Its Group Chemical Reagent Co., Ltd. (Beijing, China). The concentration of surfactants in all feed solutions was 200 mg/L. Flat PES UF membranes (SEPRO Membranes, Inc., USA) with a molecular weight cut-off (MWCO) of 5 kDa were used in experiments. The membranes were soaked in deionized water for 24 h before irradiation or filtration experiments. 2.2. Irradiation condition The membrane samples were immerged in ultrapure water in a sealed 50 mL glass tube. Gamma irradiation was applied using a 60 Co source at room temperature (ca. 250 °C) at a constant dose rate of 0.24 kGy/min. Four gamma irradiation levels were tested: 10 kGy, 25 kGy, 50 kGy and 75 kGy. The cumulated energy absorbed in the sample was measured by dosimeters with a known response to irradiation, proportional to the exposure time (Traboulsi et al., 2013). Irradiation time ranged from 41 to 312 min based on the total absorbed dose. The samples immerged in ultrapure water were stored in a refrigerator at 4 °C after irradiation. 2.3. Filtration setup A dead-end UF filtration cell connected to a 4 L feed reservoir was used for experiments as described previously (Niu et al., 2015). Feed solution from the reservoir was transferred into the UF cell using nitrogen gas at a pressure of 0.1 MPa. The filtration cell (Amicon 8200, Millipore, USA) had an effective membrane filtration area of 28.7 cm2, and a volume of 200 mL. Experiments were conducted at room temperature (25 °C) and at a stirring speed of 120 rpm. The permeate flux was measured using a balance connected to a computer. After each filtration cycle, 200 mL concentrate was generated. The water flux of the membrane was determined by filtering deionized water through the membrane until a stable permeate flux was obtained. Feed water was added to the reservoir and filtered through the membrane for 12 h. To determine the concentrations of nuclides and surfactants, filtrate of the first hour, the final concentrate, and the feed water were sampled during each experiment. 2.4. Analytical methods Concentrations of the metal ions Sr(II), Co(II), Cs(I), and Ag (I) were determined using inductively coupled plasma mass spectrometry (iCAP Q, ThermoFisher Scientific, Waltham, MA USA). The concentrations of surfactants in the solutions were determined using total organic carbon (TOC), which was measured using a Shimadzu Total Organic Carbon Analyzer (Shimadzu Corporation, Japan). Surface morphologies of the raw and fouled membranes were observed using a scanning electron microscope coupled with an energy dispersive spectrometer (SEM/EDS, Sirion 200, FEI Inc., USA). The membrane samples were freeze-dried and coated with a thin layer of gold prior to analysis. The functional groups on the membrane surfaces were detected using attenuated total reflectance - Fourier Transform Infrared Spectrometer (ATR-FTIR, Spectrum One, PerkinElmer Inc., USA). The surface roughness of the freeze-dried membrane was determined by Atomic Force Microscope (AFM, SPA-300HV, SEIKO, Japan) in tapping mode and images were processed using a NanoNavi Station (Version 5.00, SII Nano Technology Inc., Japan). The parameter Ra represents the average of the mean roughness (arithmetic average of the deviation from the center plane) of different spots and scanning sizes,
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and P-V represented the height difference between a peak and a valley. The contact angle of the membrane was measured using a contact angle analyzer (DSA100, Hamburg Germany).
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Table 1 Surface parameter and pure water flux of membrane before and after irradiation. Absorbed dose (kGy)
Ra (nm) P-V (nm) Contact angle (°)
Pure water flux (L/ (m2 h))
0 10 25 50 75
4.2 3.8 3.8 4.8 4.3
23.8 28.7 29.2 31.0 32.4
2.5. Data analysis The permeate flux (Ji, L/m2/h) was calculated as
Ji = 60 ×
mi ρ×A
(1)
where mi is the effluent mass per minute (kg/min), ρ is solution density (kg/L), and A is the outside surface area of the membrane (m2). The relative flux (RF) was calculated as
RF =
Ji J0
(2)
where J0 is pure water flux. The retention of nuclides and surfactants was calculated as
R=
cR − cP × 100% cR
(3)
where CP and CR are the concentration of pollutants in the permeate and feed solutions, respectively (μg/L for nuclides and mg/L for surfactants).
3. Results and discussion
62 124 114 128 102
92 70 70 72 68
attributed to a loss in hydrogen bonds (Sengupta et al., 2005). The FTIR data indicated that radical reactions occurring in the membrane and the network structure may have been relaxed due to a loss of hydrogen inter-chains bonds (Combernoux et al., 2015). The surface roughness was measured using AFM (Supporting information, Fig. S1) and the roughness parameters are shown in Table 1. The Ra values ranged from 3.8 nm to 4.8 nm, while the P-V values increased from 62 nm to 128 nm. No significant changes in membrane surface roughness were observed after irradiation. However, the membrane was observed to become more hydrophilic following irradiation. The contact angle of virgin membrane was 92°, compared with 68–72° after irradiation (Table 1). In accordance with these results, the pure water fluxes increased from 23.8 L/(m2 h) for the virgin membrane to 32.4 L/(m2 h) with 75 kGy-irradiated membrane. The flux was found to increase linearly with the absorbed dose. These changes are consistent with the loss in hydrogen bonds observed in the FTIR data.
3.1. Effects of irradiation on membrane surface The virgin and irradiated membrane samples were characterized using several analytical methods. The functional groups on the membrane surfaces were identified in ATR-FTIR spectra (Fig. 1). Several major peaks were observed in the spectra of virgin PES membrane, including bands at 1582 cm 1, 1491 cm 1 and 1409 cm 1, which correspond to benzene skeleton vibration, a band at 3107 cm 1 corresponding to C–H stretching in the benzene skeleton, bands at 1322 cm 1 and 1245 cm 1 corresponding to C–O–C stretching, and bands at 1151 cm 1 and 1105 cm 1 corresponding to S ¼O stretching (Tang et al., 2010). No significant changes in these major peaks were observed among the membrane samples after irradiation with different doses. However, the adsorption values at 3396 cm 1, which correspond to O-H stretching decreased after irradiation. This decrease may be
Fig. 1. ATR-FTIR spectra of membrane samples after irradiation of various doses.
3.2. Effects of irradiation on membrane performance during LLRW treatment 3.2.1. Flux variation The influence of absorbed dose on membrane performance was evaluated using surfactant-containing LLRW. The flux variations were observed to be significantly different for the irradiated membranes (Fig. 2) compared with the virgin membrane. For example, when membranes were exposed to SDBS solutions, the initial flux of irradiated membranes (ca. 60%) was considerably lower than that of the virgin membrane (73%). The normalized flux of the virgin and 10 kGy-irradiated membrane followed similar trends, with decreases of 13–20% observed during the first 200 min after which fluxes increased by 30–35% over the following 400 min. The flux variations for membranes irradiated at doses of 25–75 kGy were significantly smaller, with fluxes decreasing by 10–20% during the first 200 min and then remaining at a steady value or decreasing in the following 400 min. The initial decrease in flux may be attributed to pore blocking by surfactant monomers, while the subsequent increase may be the result of hydrophilic modification of the membrane surface by surfactant micelles. Therefore, the irradiated membranes exhibited a lower affinity for the surfactant micelles compared with the virgin membrane, while the monomer adsorption properties of pores were similar among virgin and irradiated membranes. Similar phenomena were observed with CTAB and Tween-80 solutions. The flux was found to increase for the virgin membranes, while flux decreased with irradiated membranes. At higher the absorbed doses, the flux decreased more rapidly, which may indicated a higher susceptibility of irradiated membranes to pore blocking by surfactants. The more hydrophilic surface of the irradiated membranes was less susceptible to adsorption of surfactants, while the membrane pores retained a high affinity for monomer surfactants. In summary, these results show that irradiated membranes were more easily fouled due to pore blocking.
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Fig. 3. Retention of pollutants under different conditions. Fig. 2. Flux variation of virgin and irradiated membrane.
3.2.2. Retention of pollutants The retentions of surfactants and nuclides are shown in Fig. 3. For SDBS solutions, the retentions by the virgin membrane were 70%, 97%, 97%, 91%, 60% for SDBS, Co(II), Sr(II), Ag(I) and Cs(I), respectively. For CTAB solutions, the retentions by the virgin membrane were 59%, 96%, 95%, 97%, 28% for CTAB, Co(II), Sr(II), Ag (I) and Cs(I), respectively. For Tween-80 solutions, the retentions by the virgin membrane were 70%, 29%, 32%, 28%, 27% for Tween80, Co(II), Sr(II), Ag(I) and Cs(I), respectively. The retentions of nuclides were considerably lower in nonionic surfactant solutions compared with anionic and cationic surfactant solutions. The retentions of surfactants and nuclides exhibited fluctuations of less than 10% for membranes irradiated at different doses. However, no positive or negative correlations between retention and absorbed
dose was identified. These results indicate that absorbed doses below 75 kGy had minimal effect on membrane pore sizes, since size exclusion is the key factor for UF membranes. The results are consistent with previous reports that the retention of reverse osmosis membranes remains stable at doses lower than 100 kGy (Combernoux et al., 2015). 3.2.3. Membrane fouling analysis The membrane samples fouled with SDBS solutions were analyzed using SEM/EDS (Fig. 4). Images show that deposition on the 75 kGy irradiated membrane was significantly lower than that on other membrane samples. The nuclides deposited on the membranes were not detectable by EDS for all membrane samples (Supporting information, Fig. S2). Similar results were observed for CTAB and Tween-80 samples (Supporting information, Fig. S3 and S4). The lower deposition on 75 kGy-irradiated membrane may be
X. Zhang et al. / Radiation Physics and Chemistry 127 (2016) 127–132
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Fig. 4. SEM graphs of virgin and irradiated membranes fouled by SDBS solutions.
attributed to the hydrophilic modification of membrane surface by irradiation and the lower adsorption capacity of the more hydrophilic membranes (Boussu et al., 2007). The limited deposition of surfactants on irradiated membrane surface resulted in less surface modification, as well as minimal increase in flux compared with virgin membranes.
Technology and Industry for National Defense, Changjiang Scholars and Innovative Research Team in University [IRT-13026]; and the Science & Technology Project of Tsinghua University [Grant No. 2014z21021].
Appendix A. Supporting information 4. Conclusions The resistance of PES UF membranes to irradiation by gamma rays was investigated at 0–75 kGy using a 60Co source. AFM and FTIR data showed that there were no significant changes to the membrane surfaces within the tested dose range. However, the contact angle decreased sharply from 92° to ca. 70° even at doses as low as 10 kGy. Hydrophilic modification of the membrane following irradiation may be attributed to the loss of hydrogen bond due to radical reactions at the membrane surface. In accordance with these results, the pure water flux increased linearly with the absorbed dose. The flux of the virgin membranes increased during treatment of surfactant-containing LLRW, while the flux for irradiated membranes decreased. The flux decreased more sharply in membranes exposed to higher doses. Results from SEM analysis showed that deposition on the 75 kGy-irradiated membrane was the lowest among all tested conditions, which may be attributed to higher hydrophilicity after irradiation and lower affinity for surfactant micelles. However, pore blocking by the monomers was enhanced following irradiation, leading to a decrease in flux. The retentions of nuclides and surfactants were similar for both virgin and irradiated membranes. The results show that the fouling properties of PES membrane are susceptible to gamma irradiation compared with retentions, and need to be paid more attentions. Therefore, a membrane fouling test with real or simulated wastewater is recommended to fully evaluate the membrane irradiation resistance.
Acknowledgements This work was supported by State administration of Science,
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.radphyschem. 2016.06.030.
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