- Email: [email protected]
Comparison of polyethersulfone and polyamide nanofiltration membranes for uranium removal from aqueous solution
Progress in Nuclear Energy 94 (2017) 93e100
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Comparison of polyethersulfone and polyamide nanofiltration membranes for uranium removal from aqueous solution M. Ghasemi Torkabad, A.R. Keshtkar*, S.J. Safdari Nuclear Fuel Cycle Research School, Nuclear Science and Technology Research Institute, AEOI, P.O.Box: 11365-8486, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2016 Received in revised form 13 September 2016 Accepted 24 October 2016
Dead-end filtration equipment was operated to evaluate the performance of polyethersulfone (PES-2) and polyamide (NF-1 and NF-2) membranes in terms of rejection and permeate flux for treatment of high-concentration uranium solutions under a variety of operational conditions. The optimum pH for uranium rejection using PES-2 was determined 6 while the rejection increased significantly in polyamide membranes with increase of pH. The permeate flux of all membranes increased as the pressure increased from 5 to 20 bar while the uranium rejection by these membranes changed differently. As the feed concentration increased from 7.5 to 238 mg/l, the uranium rejection by PES-2 decreased. On the contrary, the rejection by NF-1 and NF-2 increased from 57 to 79% and 62 to 98%, respectively. Also, the permeate flux of PES-2 was relatively constant whereas the permeate flux of polyamide membranes declined due to a decrease in the effective membrane pore size and an increase in osmotic pressure. The results showed that the nanofiltration process can be effectively employed for uranium removal from aqueous solutions. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Membrane Uranium Nanofiltration Polyethersulfone Polyamide
1. Introduction Concerns about longer-term security of fossil fuel supply, increase of greenhouse gases, and increasing security of energy supply have led to growth in nuclear power demand (Uranium, 2014). Therefore, demand for uranium has increased because uranium is the foundation of the nuclear power industry (Uranium, 2014; Edwards and Oliver, 2000; Nouh et al., 2015). Continuous increase in nuclear activities related to the uranium production cycle has led to higher accumulation of radioactive wastewaters containing uranium (Satpati et al., 2015). Wastewaters containing uranium are generated at all stages of the uranium production cycle that use process water and chemicals. For example, composition of a typical industrial effluent is presented in Table 1 (IAEA, 2004). Although, uranium concentration in the industrial wastewater is reported about 15 mg/l (Jansson-Charrier et al., 1996), the permissible discharge level of uranium in industry of nuclear fuel cycle changes from 0.1 to 0.5 mg/l (Anirudhan and Radhakrishnan, 2009). Therefore, in order to support treatment of uraniumcontaminated wastewaters, scientists and researchers have to
* Corresponding author. E-mail address: [email protected] (A.R. Keshtkar). http://dx.doi.org/10.1016/j.pnucene.2016.10.005 0149-1970/© 2016 Elsevier Ltd. All rights reserved.
deal with many branches of sciences and experiments (IAEA, 2004). Novel and improved methods for uranium removal from aqueous solutions are needed because of growing concern over uranium release into the environment. Also, the most common technologies applied for radioactive wastewaters treatment such as evaporation, adsorption, ion exchange, and chemical treatment have some disadvantages. For example, the main limitation of distillation (evaporation) is the its high operating cost and high energy consumption (Roach and Zapien, 2009; Sancho et al., 2006; Application of Membrane, 2004). However, the application of membrane technologies in treatment of radioactive wastewaters is increased because of various advantages over common processes (less energy consumption, modular structure, continuous performance and ease of installation and automation) (Sancho et al., 2006). Nanofiltration is a proven membrane technology which is defined as an intermediate process between reverse osmosis and ultrafiltration that provides higher fluxes than reverse osmosis at lower operating pressures (lower energy consumption) and also higher rejection of organic and inorganic solutes than ultrafiltration (Tanninen et al., 2004; Van der Bruggena et al., 2008). Recently, use of membrane technology in general and nanofiltration in particular has attracted a lot of attention in wastewaters treatment (Van der Bruggena et al., 2008).
94
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
Table 1 The composition of an industrial wastewater containing uranium (IAEA, 2004). Constituent U Th As Ra Cd Cr Mn
mg/l mg/l mg/l Bq/l mg/l mg/l mg/l
Concentration
Constituent
5.8 1.7 1 31 <0.02 0.05 41
F SO2 4 Ca Al SiO2 COD pH
Concentration mg/l mg/l mg/l mg/l mg/l mg/l
40 1560 300 90 80 102 1.54
Nanofiltration has been highly successful in removal of uranium from drinking water (Favre-Reguillon et al., 2008; Raff and Wilken, 1999), contaminated groundwater, soil and industrial wastewater (Chellam and Clifford, 2002; Rossiter et al., 2010) and even in extraction of uranium from seawater (Favre-Reguillon et al., 2003, 2005). In these researches, nanofiltration process was used for removal of uranium from solutions with low concentration (<10 mg/l). However, wastewaters and other solutions containing uranium in high concentration ranges (>10 mg/l) are also generated at some stages of the nuclear fuel cycle. For example, leach liquor of uranium ore may contain about some hundred ppm of dissolved uranium (Uranium Extraction Techno, 1993; Mishra et al., 2005). In this study, for a further understanding of the nanofiltration process for uranium removal from aqueous solutions containing uranium in high concentration ranges, three polyethersulfone and polyamide nanofiltration membranes (PES-2, NF-1, and NF-2) were tested under various operational conditions. The operational conditions of applying nanofiltration membranes play important roles in uranium rejection. These operational conditions consist of feed pH, filtration pressure, feed concentration, etc. The feed pH controls both dissociation extent of ions and surface charge density of the nanofiltration membranes. It is verified that the various nanofiltration membranes show different separation behavior in the other operational conditions. It can be said that the various observations about the effects of these operational conditions on the membranes performance has been seen in different researches (Chang et al., 2014). In this study, the effect of the following factors was studied on the membrane separation: the feed solution pH (3e9), pressure (5e20 bar), and initial feed concentration (7.5e238 mg/l). 2. Materials and methods 2.1. Feed solution A 1000 mg/l uranium stock solution was prepared by use of deionized water and analytical grade salt of UO2(NO3)2.6H2O (Merck supplied). The uranium feed solution with concentration ranges from 7.5 to 238 mg/l was prepared just before use. The feed pH was measured with a pH meter (Metrohm, Model 691) and adjusted by 1 M HNO3 and/or 1 M NaOH. 2.2. Membrane The performance of three commercially available polymeric nanofiltration membranes was investigated in this study. The basic properties of these membranes are presented in Table 2. One polyethersulfone (PES-2) and two polyamide (NF-1 and NF-2) flatsheet membranes were used. All of them were obtained from Sepro and kept as dry packets. Before being loaded into the membrane cell, the membranes were thoroughly rinsed with deionized water and soaked in a deionized water bath for 24 h.
2.3. Characterization of the membranes Commonly used pore models assume that the pores of the membranes are of the same size. If we consider the membrane pores as parallel capillaries, the permeate flux (J) can be calculated from the following equation (Al-Rashdi et al., 2013; Misra et al., 2009)
J¼
V tA
(1)
where V is the volume passed through the membrane at time t and A is the area of the membrane. By plotting pure water flux of the membrane versus variations of applied pressure (DP), the membrane permeability (pure water permeability (Lp)) can be obtained from the slope of the straight line as follows (Al-Rashdi et al., 2013):
LP ¼
J
(2)
DP
In this research, pure water flux of the membranes was measured in pressure ranges from 2.5 to 20 bar to evaluate the membrane permeability. Also, for more complete characterization of the membranes, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were obtained from 400 to 4000 cm1 using Shimadzu FTIR-8300 (Shimadzu, Japan) spectrophotometer. Moreover, membrane surface structures and the cross-sections of the membranes were investigated with scanning electron microscope (SEM) using ZEISS EVO 18 SEM (Germany). A small piece of the membranes was cut off and fractured in liquid nitrogen to examine the cross-section structure and the surface morphology. Then, the membranes were first sputter-coated with gold using a sputter-coater to make them conductive (Artug, 2007). It should be noted that the membrane samples used for characterization were rinsed thoroughly and soaked in deionized water for 24 h before they were dried in a vacuum at room temperature for two days. 2.4. Nanofiltration experimental set up The experiments were performed at room temperature in a dead-end-type cell having volume of 0.5 L and effective membrane area of 7.07 cm2 (3 cm diameter disc) with the magnetic mixer rotation rate of 500 rpm (Fig. 1). The transmembrane pressure (TMP) was obtained by applying a nitrogen static pressure. Experiments were performed under various operational conditions with an initial volume of 250 ml. By passing the solution through the membrane permeate samples were collected and the uranium concentrations were determined by an inductively coupled plasma spectroscopy (ICP, Optima, Model 7300 DV). 2.5. Uranium rejection The observed uranium rejection (R) is calculated as follows (Diallo et al., 2013):
R¼
1
CP CC
100
(3)
where CP and CC are concentrations of the permeate and retentate, respectively. CP is directly measured in the permeate fraction. CC can not be directly measured because closed membrane cell system prevents any sampling without breaking the TMP. So, this concentration is calculated from the mass balance obtained from the knowledge of the initial content in the feed, the extracted permeate volume, and the experimental permeate composition.
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
95
Table 2 Basic properties of three nanofiltration membranes tested in this study (supplied by the manufacturer). Membrane property
PES-2
NF-1
NF-2
Manufacturer Type Support material Surface material MW of Marker/MW cut-off (Dalton) Solute rejectiona (%)
Sepro Flat, polymeric Proprietary Polyethersulfone 2K PEG e
Normal operation limits Water flux (lmh)
10 bar, 50 C, and pH 2e10 e
Sepro Flat, polymeric polyester backing with a polysulfone substrate polyamide e 90 NaCl 99.5 MgSO4 83 bar, 50 C, and pH 3e10 110
Sepro Flat, polymeric polyester backing with a polysulfone substrate polyamide e 50 NaCl 98 MgSO4 83 bar, 50 C, and pH 3e10 135
a
Conditions: test pressure ¼ 10.3 bar, solute concentration ¼ 2000 mg/l, temperature ¼ 25 C.
Pressure gauge N2 Pressure
Feed Batch Magnet Permeate Stirrer Fig. 1. Dead-end filtration experimental apparatus.
3. Results and discussion 3.1. Membranes properties Pure water flux was measured as a function of transmembrane pressure for nanofiltration membranes. These measurements belong to one of the standard characterization methods. In all three membranes, pressure increase from 2.5 to 20 bar led to a linear volumetric water flux increase. Fig. 2 shows dependence of the volumetric water flux on the transmembrane pressure for different nanofiltration membranes. Using Eq. (2), the permeability of membranes (Lp) were found to be 157.17, 5.61 and 25.06 l/(hm2bar) for PES-2, NF-1, and NF-2, respectively. Three membranes showed generally a linear variation of their water flux versus transmembrane pressure (R2 ¼ 0.99).
Fig. 2. Dependence of the volumetric water flux on the transmembrane pressure for nanofiltration membranes (PES-2, NF-1, and NF-2).
The results of the transmembrane pressure effect on the water flux suggest that the NF-1 and PES-2 membranes had the most and the least compact structures, respectively while structural compaction of NF-2 was in the middle of two other membranes. Moreover, Artug and Hapke (Artug and Hapke, 2006) characterized some nanofiltration membranes on the basis of their morphology and charge parameters and investigated the influence of these parameters on the filtration performance. They found that the NF-2 membrane had permeability of about 17 l/(hm2bar). Both of these permeability results indicate that the NF-2 membrane is neither a loose nor a tight nanofiltration membrane. It should be noted that the obtained results of the permeate fluxes of NF-1 and NF-2 membranes in this research and the specified rate of the fluxes given for these membranes in Table 2 are different. It can be said that the permeability of the nanofiltration membrane is very sensitive to purity and other conditions of the solution (water). Both of them are experimental data which are achieved in relatively different conditions. It is obvious that sometimes there is a clear difference between two membrane experiments (even if the conditions are the same). However, different sections of NF-1 and NF-2 membranes were used in each experiment. Thus, the differences in the results may indicate heterogeneity of the membranes surfaces. The use of different samples of membrane in each experiment may represent nearly the actual permeate flux of the membranes (Al-Rashdi et al., 2013). In order to verify the structural information of PES-2, NF-1, and NF-2 membranes, ATR-FTIR spectra analysis was used. Characteristic groups of polyethersulfone and polyamide can be seen in all the membranes in Fig. 3. In other researches (Diallo et al., 2013; Singh et al., 2006), similar characteristic groups were detected for polyethersulfone and polyamide membranes. Significant differences among three membranes were not be discernable just only the bands at ~1535 and 1502 cm1 in the NF-1 and NF-2
Fig. 3. ATR-FTIR spectra of PES-2, NF-1, and NF-2 membranes.
96
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
membranes. From the ATR-FTIR spectra (Fig. 3), no difference between these three membranes could be perceived. However, our experiment results showed that the PES-2, NF-1, and NF-2 membranes performances were different, especially in uranium rejection. These differences, to the best of our knowledge, are due to the proprietary layer structure. Understanding details of relationship between the material of the membranes and the membrane performance needs further study. In general, all the membranes had very similar spectra peaks. The similarity of spectra shows that these membranes have the same basic structure. Basic properties of three nanofiltration membranes that were supplied by the manufacturer are presented in Table 2. According to Table 2, the structural materials of the membranes are similar. Therefore, the obtained results from the ATR-FTIR analysis are reasonable. In another research (Singh et al., 2006), similar results were found for polyethersulfone and polyamide membranes. SEM pictures of the surface and the cross-section of the membranes are shown in Fig. 4. Comparison of the surface structures shows that the NF-1 membrane has a relatively rough surface
(surface pores). Moreover, layer structures of the membranes are distinguishable from the cross-section views and the skin layers of the nanofiltration membranes have thicknesses below 100 nm (Yaroshchuk et al., 2013). There is a correlation between surface roughness and membrane fouling (nanofiltration membrane with a rough surface is more prone to fouling than nanofiltration membrane with a smoother surface) (Van der Bruggena et al., 2008; Artug and Hapke, 2006). The NF-1 membrane has higher surface porosity than the PES-2 and NF-2 membranes but water permeability of this membrane is less than two other membranes (Fig. 2). Since the hydrophobic nanofiltration membranes have lower permeability, this finding may be ascribed to higher hydrophobic property of NF-1 membrane than the PES-2 and NF-2 membranes (Artug and Hapke, 2006). Therefore, fouling propensity of NF-1 membrane is more than two other membranes because of surface roughness and hydrophobicity. Membrane fouling may be reduced by applying membranes with lower surface roughness and higher hydrophilicity of the membrane surface (Van der Bruggena et al., 2008).
Fig. 4. SEM pictures of the (right) surface and (left) cross-section of PES-2, NF-1, and NF-2 membranes.
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
3.2. Effect of feed pH on membranes performance The solution pH is a very important factor which can change the nature of the membrane surface charge and pore size, as well as type and concentration of the dissolved metal species and consequently can influence the membrane separation performance (AlRashdi et al., 2013; Gherasim et al., 2013). Therefore, the solution pH is often the key parameter to control the desired separation (Van der Bruggena et al., 2008). Fig. 5 presents the effect of pH on the rejection of uranium and the permeate flux in three nanofiltration membranes at operating pressure of 10 bar and 15 mg/l uranium feed solution. It was found that the effects of feed pH on performance of polyethersulfone nanofiltration membrane (PES-2) and polyamide nanofiltration membranes (NF-1 and NF-2) were different. The results showed that the permeate flux of PES-2 membrane increased with an increase in the pH ranges of 4e6. Moreover, the rejection of uranium increased with increasing the pH from 4 to 6. Further increase of pH from 6 to 9 resulted in the decrease of
97
permeate flux and the rejection of uranium. This phenomenon could be attributed to the isoelectric point of PES-2 membrane at the pH 6 since the maximum permeate flux and rejection were observed around the isoelectric point of some nanofiltration membranes (Al-Rashdi et al., 2013; Shang et al., 2014). Therefore, the optimum pH for uranium removal by means of PES-2 nanofiltration membrane was determined 6. A similar trend was observed by Shang et al. (2014). They observed that permeate flux increased with an increase in the pH ranges of 2.5e6.5 while the rejection of vanadium remained constant. More increase of pH led to a decrease in the permeate flux and especially the rejection of vanadium. So, they reported that pH 6e6.5 is the optimum pH for concentrating vanadium from acid leach solution using DL and DK nanofiltration membranes. The other researchers explained that the pore size of the nanofiltration membranes changes at the isoelectric pH due to the expanding or shrinking of the skin layer of the membrane, the lowest electroviscous effect, and the highest net driving force in consequence of lowest osmotic pressure at the surface of the membrane (Al-Rashdi et al., 2013). For NF-1 and NF-2 membranes, the uranium rejection increased significantly within the pH ranges from 3 to 9. For both membranes the permeate flux first decreased and then remained constant for NF-1 while a little increase observed for NF-2 as the feed pH increased. Membrane charge density of negatively charged membrane also rises with increase of pH due to the dissociation of acidic groups which is reflected as an increase in salt rejection (Artug, 2007). Thus, these results indicated that NF-1 and NF-2 are negatively charged membranes. A speciation distribution was determined with the aid of the Visual Minteq software. The obtained results showed that the predicted species in a solution containing 16.2 mg/l UO2þ and 2 þ2 þ 7.44 mg/l NO 3 at pH ranges of 3e4 were UO2 and UO2OH . The þ complexes of (UO2)3(OH)þ 5 and (UO2)4(OH)7 dominated the system in pH ranges of 5e7, but at pH 9, (UO2)3(OH)-7 and UO2(OH)-3 were the major species. The more general feature of nanofiltration membranes is the separation of ions according to their size and valency (Favre-Reguillon et al., 2005). Thus, a high rejection coefficient of this bulky and highly charged uranium species can be expected. Briefly, small uranyl complexes were predominant within low pH ranges while the big complexes had been dominated by increasing the pH. Moreover, a big complex has a bigger hydrated radius than the small uranyl complexes. Therefore, higher rate of uranium was rejected at higher pHs. According to the obtained results, the PES-2 membrane had the highest permeate flux and the lowest uranium rejection because the PES-2 membrane was a loose nanofiltration membrane. Also, the uranium rejection by NF-1 membrane was higher than that of NF-2 membrane in the pH ranges of 3e4 but uranium rejection by NF-1 membrane was lower than that of NF-2 membrane with an increase in the pH ranges of 6e9. On the other hand, the NF-1 membrane had lower permeate flux than the NF-2 membrane in the studied pH range (3e9). This result confirmed that both of the charge exclusion and steric exclusion are important for uranium rejection in the NF-1 and NF-2 membranes. Anyway, surface structures of these two nanofiltration membranes were different (according to the SEM pictures), hence the surface charge behavior could be relatively different between these membranes with the same construction materials. 3.3. Effect of applied pressure on membranes performance
Fig. 5. Effect of pH on the rejection of uranium and the permeate flux of PES-2, NF-1, and NF-2 membranes at operating pressure 10 bar for 15 mg/l uranium feed solution.
Uranium rejection and permeate flux as a function of the operating pressure are shown in Fig. 6. These results were related to feed solutions with 15 mg/l uranium concentration at pH 6. For all
98
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
Fig. 6. Effect of operating pressure on the rejection of uranium and the permeate flux of PES-2, NF-1, and NF-2 membranes for feed solutions with 15 mg/l uranium concentrations at pH 6.
membranes the permeate flux increased with increasing the pressure from 5 to 20 bar while the uranium rejection by these membranes followed different trends. Generally, the pressure effect can be described by two mechanisms which control the uranium separation. First, an increase in the transmembrane pressure leads to an increase in the water flux but the uranyl ions transport is hindered by the steric and charge effects (the ion flux and water flux are uncoupled). Therefore, the higher water flux results in a dilution of permeate and consequently a higher rejection. Second, increasing the pressure will cause more uranyl ions transport to the membrane surface and, so the concentration polarization will increase which leads to a decrease in uranium rejection by decreasing the charge effect. The results of these two mechanisms that occur simultaneously will determine the rejection behavior at different transmembrane pressures (Gherasim et al., 2013; Fang and Deng, 2014). The overall effect of operating pressure on uranium rejection is controlled by these two competing mechanisms. The results of PES-2 membrane
indicated that the more ions transporting toward the membrane surface plays a dominant role. An increase in uranium rejection by NF-1 membrane as the pressures changes from 5 to 10 bar is generally explained by a shift in the transport mechanism across the membrane. At lower pressures a diffusive transport of uranium occurs which accounts for the lower rejections while convective transport of uranyl ions through the membrane is dominant mechanism at pressure 10 bar. When the applied pressure increases further, the uranium rejections decrease because the concentration polarization effects start to become significant. As mentioned before, concentration polarization which increases with increasing the pressure, decreases the rejection albeit the convective transport results in an increase in rejection. The counteracting contributions of the increased convective transport and the increased concentration polarization resulted in a nearly constant uranium rejection value for NF-2 membrane. Accordingly, in terms of uranium rejection, NF-2 had relatively stable behavior in spite of pressure changes and the behavior of NF-2 was different from the other two membranes. The increase of the permeate fluxes of all of these membranes with transmembrane pressure increase indicates that an increase in operating pressure will enhance the driving force and then will overcome the membrane resistance (Zuo et al., 2008). Also, as the pressure increases, convective transport and concentration polarization become more significant (Al-Rashdi et al., 2013). Fig. 6 shows liner change in the permeate flux with increasing the pressure. Although a similar and undeniable result was observed in the case of arsenic removal using a commercial nanofiltration membrane (NF270) in another research (Al-Rashdi et al., 2013), the increase in permeate flux of NF270 membrane was not linear for copper, lead and manganese with the rise of pressure which showed a quantity of concentration polarization. Moreover, the permeate fluxes at pressure 10 bar for PES-2, NF-1, and NF-2 membranes are 1698, 45, and 212 l/(hm2), respectively (Fig. 6). These results are lower than the corresponding pure water flux, 2314, 62, and 286 l/(hm2) for PES-2, NF-1, and NF-2, respectively (Fig. 2). This can be taken as an indication of membrane fouling and/or osmotic pressure build up caused by the retained salt. A similar reduction in the permeate flux was observed by Al-Rashdi et al. (2013). It should be noted that increase of the permeate flux of membrane leads the concentration polarization effects to become exponentially larger. In the laboratory systems, concentration polarization is controlled by increasing the bulk mixing of the feed solution. But, there are some practical limitations for this approach in industrial systems. Improving design of membrane module and developing methods of controlling the feed solution flow in the membrane module are the best solution for minimizing the effects of concentration polarization (Baker, 2004). Also, the concentration polarization could be quantified by the degree of concentration polarization which is defined as the ratio of the solute concentration at the surface of membrane and the solute concentration in the bulk of solution (Bi et al., 2014). In this research, it was tried to reduce the effects of this phenomenon by bulk mixing of the feed solution. The obtained results showed that there was a quantity of concentration polarization as it could be indicated by the nonlinear relationship between uranium rejection and pressure. 3.4. Effect of feed concentration on membranes performance Membranes performances in terms of the filtration flux and uranium rejection efficiency were studied by varying the feed uranium concentration from 7.5 to 238 mg/l at feed pH 6. The operating pressure was held constant at 10 bar. Fig. 7 shows the results of PES-2, NF-1, and NF-2 membranes. It was found that the
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
99
The rejection of uranium by polyamide nanofiltration membranes increased with an increase in initial concentration. The uranium rejection by NF-1 membrane changed from 57 to 79% as the initial uranium concentration increased from 7.5 to 238 mg/l. Similarly, the uranium rejection by NF-2 membrane increased from 62 to 98%. The increase of the uranium rejection with increase of initial uranium concentration suggested that the transport of uranium through the membrane did not increase proportional to the increase of uranium concentration. The increase of solute rejection with increment of feed concentration can be explained by the adsorption of the solute on the membrane surface (Al-Rashdi et al., 2013; Chung et al., 2005). There are two reasons for this claim. First, the compact membranes of NF-1 and NF-2 would be expected to adsorb higher amount of the solute on the membrane surface due to smaller pore size than the wide-open structure PES-2 membrane. Second, the operational pH for all experiments was adjusted at 6. As be noted before, isoelectric point of PES-2 membrane was 6 and thus the surface charge of this membrane was zero at pH 6. However, the NF-1 and NF-2 membranes had negative surface charge and the positive complexes of (UO2)3(OH)þ and 5 (UO2)4(OH)þ 7 were the dominant species of uranium at this pH. Therefore, on the basis of above discussions, adsorption of the uranium complexes on the surface of polyamide membranes is reasonable. It is interesting that the uranium rejection rate by NF-1 membrane was significantly lower than the NF-2 membrane. Probably the charge exclusion plays a critical role for uranium rejection at high initial uranium concentration rather than the steric exclusion. So, the NF-2 membrane has higher uranium rejection than the NF-1 membrane while NF-1 has the more compact structure than NF-2. It is obvious from Fig. 7 that the permeate flux of PES-2 is relatively constant but the permeate flux of polyamide nanofiltration membranes declines with increase of uranium concentration. These results can be described by two phenomena. First, adsorption or deposition of solute on the membrane surface leads to a decrease in the effective membrane pore size. Second, the increase of feed concentration leads to an increase in osmotic pressure (IAEA, 2004; Al-Rashdi et al., 2013). Accordingly, permeate flux decreased with increasing the feed concentration. 4. Conclusions Fig. 7. Effect of feed concentration on the rejection of uranium and the permeate flux of PES-2, NF-1, and NF-2 membranes at operating pressure 10 bar and pH 6.
effects of feed concentration on performance of polyethersulfone nanofiltration membrane (PES-2) and polyamide nanofiltration membranes (NF-1 and NF-2) were different. Because of the wide open structure, the rejection of PES-2 membrane was lower than two other membranes. As the feed uranium concentration increased, the uranium rejection of PES-2 decreased. The increased feed uranium concentrations (from 15 to 238 mg/l) caused about 90% decrease in uranium rejection efficiency. The downward trend was also reported for NF270 membrane as metal (Cd(II), Cu(II) and Pb(II)) concentrations increased from 100 to 2000 mg/l (Al-Rashdi et al., 2013). There are two phenomena by which increasing the concentration of ions on the membrane surface affect ions rejection. As the concentration increases, the electrostatic forces between the membrane and the ions in solution begin to weaken. Also, by increase of the concentration driving force for ions transport the diffusive factor of the transport through the membrane increased (IAEA, 2004; Chang et al., 2014). Both of these can lead to the decrease of uranium rejection.
Uranium removal from high-concentration aqueous solutions was studied by using three polyethersulfone (PES-2) and polyamide (NF-1 and NF-2) nanofiltration membranes under various operational conditions such as feed pH, transmembrane pressure, and uranium concentration of feed solution. It was found that the pH of the feed solution affects both uranium rejection and permeate flux because the charge properties of surface layer of the nanofiltration membranes and dissolved uranium species changed with pH. Increase of transmembrane pressure led to the permeate flux increase and the overall effect of operating pressure on the uranium rejection could be controlled by two competing mechanisms. Feed concentration affected the rejection and the permeate flux. For example, the uranium rejection by NF-1 and NF-2 membranes increased, whereas the permeate flux slightly decreased with an increase in feed solution concentration. The PES-2 membrane had higher permeate fluxes than the NF-1 and NF-2 membranes whereas uranium rejections by PES-2 membrane was lower than that of NF-1 and NF-2. Generally, the results of rejection and permeate flux showed that NF-2 polyamide membrane had the best performance compared to two other membranes. In previous researches, nanofiltration process was used for treatment of uranium-contaminated solutions with low concentration. But, wastewaters containing uranium in high concentration ranges are
100
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
generated at some stages of the nuclear fuel cycle and there is the possibility of uranium release into the environment. Our research proves that the nanofiltration process, as a non-polluting separation method, can be effectively used for uranium removal and recovery from these solutions. Because, this method has desirable performance for uranium rejection and some unique advantages such as less energy consumption, modular structure, continuous performance, and ease of installation and automation over common technologies that used for treatment of radioactive wastewaters. Also, this method will provide some reductions including the amount of raw materials required, the effluent produced, the size of plant, and the overall related cost. References Al-Rashdi, B.A.M., Johnson, D.J., Hilal, N., 2013. Removal of heavy metal ions by nanofiltration. Desalination 315, 2e17. Anirudhan, T.S., Radhakrishnan, P.G., 2009. Improved performance of a biomaterialbased cation exchanger for the adsorption of uranium(VI) from water and nuclear industry wastewater. J. Environ. Radioact. 100, 250e257. Application of Membrane Technologies for Liquid Radioactive Waste Processing, 2004. IAEA, Vienna, p. 145. Artug, G., 2007. Modelling and Simulation of Nanofiltration Membranes. Cuvillier €ttingen, p. 248. Verlag, Go Artug, G., Hapke, J., 2006. Characterization of nanofiltration membranes by their morphology, charge and filtration performance parameters. Desalination 200, 178e180. Baker, R.W., 2004. Membrane Technology and Applications. John Wiley & Sons Ltd, p. 538. Bi, F., Zhao, H., Zhang, L., Ye, Q., Chen, H., Gao, C., 2014. Discussion on calculation of maximum water recovery in nanofiltration system. Desalination 332, 142e146. Chang, F., Liu, W., Wang, X., 2014. Comparison of polyamide nanofiltration and lowpressure reverse osmosis membranes on As(III) rejection under various operational conditions. Desalination 334, 10e16. Chellam, S., Clifford, D.A., 2002. Physicalechemical treatment of groundwater contaminated by leachate from surface disposal of uranium tailings. J. Environ. Eng. 128, 942e952. Chung, C.V., Buu, N.Q., Chau, N.H., 2005. Influence of surface charge and solution pH on the performance characteristics of a nanofiltration membrane. Sci. Technol. Adv. Mater. 6, 246e250. Diallo, H., Rabiller-Baudry, M., Khaless, K., Chaufer, B., 2013. On the electrostatic interactions in the transfer mechanisms of iron during nanofiltration in high concentrated phosphoric acid. J. Membr. Sci. 427, 37e47. Edwards, C.R., Oliver, A.J., 2000. Uranium processing: a review of current methods and technology. JOM 52, 12e20. Fang, J., Deng, B., 2014. Rejection and modeling of arsenate by nanofiltration: contributions of convection, diffusion and electromigration to arsenic transport. J. Membr. Sci. 453, 42e51. Favre-Reguillon, A., Lebuzit, G., Foos, J., Guy, A., Draye, M., Lemaire, M., 2003. Selective concentration of uranium from seawater by nanofiltration. Ind. Eng. Chem. Res. 42, 5900e5904. Favre-Reguillon, A., Lebuzit, G., Foos, J., Guy, A., Sorin, A., Lemaire, M., Draye, M., 2005. Selective rejection of dissolved uranium carbonate from seawater using
cross-flow filtration technology. Sep. Sci. Technol. 40, 623e631. Favre-Reguillon, A., Lebuzit, G., Murat, D., Foos, J., Mansour, C., Draye, M., 2008. Selective removal of dissolved uranium in drinking water by nanofiltration. Water Res. 42, 1160e1166. sek, P., 2013. Analysis of lead(II) retention from Gherasim, C.V., Cuhorka, J., Mikula single salt and binary aqueous solutions by apolyamide nanofiltration membrane: experimental results and modelling. J. Membr. Sci. 436, 132e144. Treatment of Liquid Effluent from Uranium Mines and Mills, 2004. IAEA, pp. 27e44. Jansson-Charrier, M., Guibal, E., Roussy, J., Surjous, R., Le Cloirec, P., 1996. Dynamic removal of uranium by chitosan: influence of operating parameters. Water Sci. Technol. 34, 169e177. Mishra, S.L., Vijayalakshmi, R., Singh, H., 2005. Extraction of uranium from acidic media with a novel synergistic mixture of di-nonyl phenyl phosphoric acid and tri-n-octyl phosphine oxide. Indian J. Chem. Technol. 12, 708e712. Misra, S.K., Mahatele, A.K., Tripathi, S.C., Dakshinamoorthy, A., 2009. Studies on the simultaneous removal of dissolved DBP and TBP as well as uranyl ions from aqueous solutions by using Micellar-Enhanced Ultrafiltration Technique. Hydrometallurgy 96, 47e51. Nouh, E.S.A., Amin, M., Gouda, M., Abd-Elmagid, A., 2015. Extraction of uranium(VI) from sulfate leach liquor after iron removal using manganese oxide coated zeolite. J. Environ. Chem. Eng. 3, 523e528. Raff, O., Wilken, R.D., 1999. Removal of dissolved uranium by nanofiltration. Desalination 122, 147e150. Roach, J.D., Zapien, J.H., 2009. Inorganic ligand-modified, colloid-enhanced ultrafiltration: a novel method for removing uranium from aqueous solution. Water Res. 43, 4751e4759. €fer, A.I., 2010. Impact of speciation on behaviour Rossiter, H.M.A., Graham, M.C., Scha of uranium in a solar powered membrane system for treatment of brackish groundwater. Sep. Purif. Technol. 71, 89e96. Sancho, M., Arnal, J.M., Verdú, G., Lora, J., Villaescusa, J.I., 2006. Ultrafiltration and reverse osmosis performance in the treatment of radioimmunoassay liquid wastes. Desalination 201, 207e215. Satpati, S.K., Roy, S.B., Pal, S., Tewari, P.K., 2015. Development of methodology for separation and recovery of uranium from nuclear wastewater. BARC Newsl. 347, 23e29. Shang, G., Zhang, G., Gao, C., Fu, W., Zeng, L., 2014. A novel nanofiltration process for the recovery of vanadium from acid leach solution. Hydrometallurgy 142, 94e97. Singh, P.S., Joshi, S.V., Trivedi, J.J., Devmurari, C.V., Rao, A.P., Ghosh, P.K., 2006. Probing the structural variations of thin film composite RO membranes obtained by coating polyamide over polysulfone membranes of different pore dimensions. J. Membr. Sci. 278, 19e25. €m, M., 2004. Long-term acid resistance and Tanninen, J., Platt, S., Weis, A., Nystro selectivity of NF membranes in very acidic conditions. J. Membr. Sci. 240, 11e18. Uranium 2014: Resources, production and demand, 2014. OECD Nuclear Energy Agency, France, p. 504. Uranium Extraction Technology, 1993. IAEA, Vienna, p. 380. Van der Bruggena, B., Manttari, M., Nystrom, M., 2008. Drawbacks of applying nanofiltration and how to avoid them: a review. Sep. Purif. Technol. 63, 251e263. Yaroshchuk, A., Bruening, M.L., Bernal, E.E.L., 2013. Solution-Diffusion-ElectroMigration model and its uses for analysis of nanofiltration, pressure-retarded osmosis and forward osmosis in multi-ionicsolutions. J. Membr. Sci. 447, 463e476. Zuo, W., Zhang, G., Meng, Q., Zhang, H., 2008. Characteristics and application of multiple membrane process in plating wastewater reutilization. Desalination 222, 187e196.
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Comparison of polyethersulfone and polyamide nanofiltration membranes for uranium removal from aqueous solution M. Ghasemi Torkabad, A.R. Keshtkar*, S.J. Safdari Nuclear Fuel Cycle Research School, Nuclear Science and Technology Research Institute, AEOI, P.O.Box: 11365-8486, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2016 Received in revised form 13 September 2016 Accepted 24 October 2016
Dead-end filtration equipment was operated to evaluate the performance of polyethersulfone (PES-2) and polyamide (NF-1 and NF-2) membranes in terms of rejection and permeate flux for treatment of high-concentration uranium solutions under a variety of operational conditions. The optimum pH for uranium rejection using PES-2 was determined 6 while the rejection increased significantly in polyamide membranes with increase of pH. The permeate flux of all membranes increased as the pressure increased from 5 to 20 bar while the uranium rejection by these membranes changed differently. As the feed concentration increased from 7.5 to 238 mg/l, the uranium rejection by PES-2 decreased. On the contrary, the rejection by NF-1 and NF-2 increased from 57 to 79% and 62 to 98%, respectively. Also, the permeate flux of PES-2 was relatively constant whereas the permeate flux of polyamide membranes declined due to a decrease in the effective membrane pore size and an increase in osmotic pressure. The results showed that the nanofiltration process can be effectively employed for uranium removal from aqueous solutions. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Membrane Uranium Nanofiltration Polyethersulfone Polyamide
1. Introduction Concerns about longer-term security of fossil fuel supply, increase of greenhouse gases, and increasing security of energy supply have led to growth in nuclear power demand (Uranium, 2014). Therefore, demand for uranium has increased because uranium is the foundation of the nuclear power industry (Uranium, 2014; Edwards and Oliver, 2000; Nouh et al., 2015). Continuous increase in nuclear activities related to the uranium production cycle has led to higher accumulation of radioactive wastewaters containing uranium (Satpati et al., 2015). Wastewaters containing uranium are generated at all stages of the uranium production cycle that use process water and chemicals. For example, composition of a typical industrial effluent is presented in Table 1 (IAEA, 2004). Although, uranium concentration in the industrial wastewater is reported about 15 mg/l (Jansson-Charrier et al., 1996), the permissible discharge level of uranium in industry of nuclear fuel cycle changes from 0.1 to 0.5 mg/l (Anirudhan and Radhakrishnan, 2009). Therefore, in order to support treatment of uraniumcontaminated wastewaters, scientists and researchers have to
* Corresponding author. E-mail address: [email protected] (A.R. Keshtkar). http://dx.doi.org/10.1016/j.pnucene.2016.10.005 0149-1970/© 2016 Elsevier Ltd. All rights reserved.
deal with many branches of sciences and experiments (IAEA, 2004). Novel and improved methods for uranium removal from aqueous solutions are needed because of growing concern over uranium release into the environment. Also, the most common technologies applied for radioactive wastewaters treatment such as evaporation, adsorption, ion exchange, and chemical treatment have some disadvantages. For example, the main limitation of distillation (evaporation) is the its high operating cost and high energy consumption (Roach and Zapien, 2009; Sancho et al., 2006; Application of Membrane, 2004). However, the application of membrane technologies in treatment of radioactive wastewaters is increased because of various advantages over common processes (less energy consumption, modular structure, continuous performance and ease of installation and automation) (Sancho et al., 2006). Nanofiltration is a proven membrane technology which is defined as an intermediate process between reverse osmosis and ultrafiltration that provides higher fluxes than reverse osmosis at lower operating pressures (lower energy consumption) and also higher rejection of organic and inorganic solutes than ultrafiltration (Tanninen et al., 2004; Van der Bruggena et al., 2008). Recently, use of membrane technology in general and nanofiltration in particular has attracted a lot of attention in wastewaters treatment (Van der Bruggena et al., 2008).
94
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
Table 1 The composition of an industrial wastewater containing uranium (IAEA, 2004). Constituent U Th As Ra Cd Cr Mn
mg/l mg/l mg/l Bq/l mg/l mg/l mg/l
Concentration
Constituent
5.8 1.7 1 31 <0.02 0.05 41
F SO2 4 Ca Al SiO2 COD pH
Concentration mg/l mg/l mg/l mg/l mg/l mg/l
40 1560 300 90 80 102 1.54
Nanofiltration has been highly successful in removal of uranium from drinking water (Favre-Reguillon et al., 2008; Raff and Wilken, 1999), contaminated groundwater, soil and industrial wastewater (Chellam and Clifford, 2002; Rossiter et al., 2010) and even in extraction of uranium from seawater (Favre-Reguillon et al., 2003, 2005). In these researches, nanofiltration process was used for removal of uranium from solutions with low concentration (<10 mg/l). However, wastewaters and other solutions containing uranium in high concentration ranges (>10 mg/l) are also generated at some stages of the nuclear fuel cycle. For example, leach liquor of uranium ore may contain about some hundred ppm of dissolved uranium (Uranium Extraction Techno, 1993; Mishra et al., 2005). In this study, for a further understanding of the nanofiltration process for uranium removal from aqueous solutions containing uranium in high concentration ranges, three polyethersulfone and polyamide nanofiltration membranes (PES-2, NF-1, and NF-2) were tested under various operational conditions. The operational conditions of applying nanofiltration membranes play important roles in uranium rejection. These operational conditions consist of feed pH, filtration pressure, feed concentration, etc. The feed pH controls both dissociation extent of ions and surface charge density of the nanofiltration membranes. It is verified that the various nanofiltration membranes show different separation behavior in the other operational conditions. It can be said that the various observations about the effects of these operational conditions on the membranes performance has been seen in different researches (Chang et al., 2014). In this study, the effect of the following factors was studied on the membrane separation: the feed solution pH (3e9), pressure (5e20 bar), and initial feed concentration (7.5e238 mg/l). 2. Materials and methods 2.1. Feed solution A 1000 mg/l uranium stock solution was prepared by use of deionized water and analytical grade salt of UO2(NO3)2.6H2O (Merck supplied). The uranium feed solution with concentration ranges from 7.5 to 238 mg/l was prepared just before use. The feed pH was measured with a pH meter (Metrohm, Model 691) and adjusted by 1 M HNO3 and/or 1 M NaOH. 2.2. Membrane The performance of three commercially available polymeric nanofiltration membranes was investigated in this study. The basic properties of these membranes are presented in Table 2. One polyethersulfone (PES-2) and two polyamide (NF-1 and NF-2) flatsheet membranes were used. All of them were obtained from Sepro and kept as dry packets. Before being loaded into the membrane cell, the membranes were thoroughly rinsed with deionized water and soaked in a deionized water bath for 24 h.
2.3. Characterization of the membranes Commonly used pore models assume that the pores of the membranes are of the same size. If we consider the membrane pores as parallel capillaries, the permeate flux (J) can be calculated from the following equation (Al-Rashdi et al., 2013; Misra et al., 2009)
J¼
V tA
(1)
where V is the volume passed through the membrane at time t and A is the area of the membrane. By plotting pure water flux of the membrane versus variations of applied pressure (DP), the membrane permeability (pure water permeability (Lp)) can be obtained from the slope of the straight line as follows (Al-Rashdi et al., 2013):
LP ¼
J
(2)
DP
In this research, pure water flux of the membranes was measured in pressure ranges from 2.5 to 20 bar to evaluate the membrane permeability. Also, for more complete characterization of the membranes, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were obtained from 400 to 4000 cm1 using Shimadzu FTIR-8300 (Shimadzu, Japan) spectrophotometer. Moreover, membrane surface structures and the cross-sections of the membranes were investigated with scanning electron microscope (SEM) using ZEISS EVO 18 SEM (Germany). A small piece of the membranes was cut off and fractured in liquid nitrogen to examine the cross-section structure and the surface morphology. Then, the membranes were first sputter-coated with gold using a sputter-coater to make them conductive (Artug, 2007). It should be noted that the membrane samples used for characterization were rinsed thoroughly and soaked in deionized water for 24 h before they were dried in a vacuum at room temperature for two days. 2.4. Nanofiltration experimental set up The experiments were performed at room temperature in a dead-end-type cell having volume of 0.5 L and effective membrane area of 7.07 cm2 (3 cm diameter disc) with the magnetic mixer rotation rate of 500 rpm (Fig. 1). The transmembrane pressure (TMP) was obtained by applying a nitrogen static pressure. Experiments were performed under various operational conditions with an initial volume of 250 ml. By passing the solution through the membrane permeate samples were collected and the uranium concentrations were determined by an inductively coupled plasma spectroscopy (ICP, Optima, Model 7300 DV). 2.5. Uranium rejection The observed uranium rejection (R) is calculated as follows (Diallo et al., 2013):
R¼
1
CP CC
100
(3)
where CP and CC are concentrations of the permeate and retentate, respectively. CP is directly measured in the permeate fraction. CC can not be directly measured because closed membrane cell system prevents any sampling without breaking the TMP. So, this concentration is calculated from the mass balance obtained from the knowledge of the initial content in the feed, the extracted permeate volume, and the experimental permeate composition.
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
95
Table 2 Basic properties of three nanofiltration membranes tested in this study (supplied by the manufacturer). Membrane property
PES-2
NF-1
NF-2
Manufacturer Type Support material Surface material MW of Marker/MW cut-off (Dalton) Solute rejectiona (%)
Sepro Flat, polymeric Proprietary Polyethersulfone 2K PEG e
Normal operation limits Water flux (lmh)
10 bar, 50 C, and pH 2e10 e
Sepro Flat, polymeric polyester backing with a polysulfone substrate polyamide e 90 NaCl 99.5 MgSO4 83 bar, 50 C, and pH 3e10 110
Sepro Flat, polymeric polyester backing with a polysulfone substrate polyamide e 50 NaCl 98 MgSO4 83 bar, 50 C, and pH 3e10 135
a
Conditions: test pressure ¼ 10.3 bar, solute concentration ¼ 2000 mg/l, temperature ¼ 25 C.
Pressure gauge N2 Pressure
Feed Batch Magnet Permeate Stirrer Fig. 1. Dead-end filtration experimental apparatus.
3. Results and discussion 3.1. Membranes properties Pure water flux was measured as a function of transmembrane pressure for nanofiltration membranes. These measurements belong to one of the standard characterization methods. In all three membranes, pressure increase from 2.5 to 20 bar led to a linear volumetric water flux increase. Fig. 2 shows dependence of the volumetric water flux on the transmembrane pressure for different nanofiltration membranes. Using Eq. (2), the permeability of membranes (Lp) were found to be 157.17, 5.61 and 25.06 l/(hm2bar) for PES-2, NF-1, and NF-2, respectively. Three membranes showed generally a linear variation of their water flux versus transmembrane pressure (R2 ¼ 0.99).
Fig. 2. Dependence of the volumetric water flux on the transmembrane pressure for nanofiltration membranes (PES-2, NF-1, and NF-2).
The results of the transmembrane pressure effect on the water flux suggest that the NF-1 and PES-2 membranes had the most and the least compact structures, respectively while structural compaction of NF-2 was in the middle of two other membranes. Moreover, Artug and Hapke (Artug and Hapke, 2006) characterized some nanofiltration membranes on the basis of their morphology and charge parameters and investigated the influence of these parameters on the filtration performance. They found that the NF-2 membrane had permeability of about 17 l/(hm2bar). Both of these permeability results indicate that the NF-2 membrane is neither a loose nor a tight nanofiltration membrane. It should be noted that the obtained results of the permeate fluxes of NF-1 and NF-2 membranes in this research and the specified rate of the fluxes given for these membranes in Table 2 are different. It can be said that the permeability of the nanofiltration membrane is very sensitive to purity and other conditions of the solution (water). Both of them are experimental data which are achieved in relatively different conditions. It is obvious that sometimes there is a clear difference between two membrane experiments (even if the conditions are the same). However, different sections of NF-1 and NF-2 membranes were used in each experiment. Thus, the differences in the results may indicate heterogeneity of the membranes surfaces. The use of different samples of membrane in each experiment may represent nearly the actual permeate flux of the membranes (Al-Rashdi et al., 2013). In order to verify the structural information of PES-2, NF-1, and NF-2 membranes, ATR-FTIR spectra analysis was used. Characteristic groups of polyethersulfone and polyamide can be seen in all the membranes in Fig. 3. In other researches (Diallo et al., 2013; Singh et al., 2006), similar characteristic groups were detected for polyethersulfone and polyamide membranes. Significant differences among three membranes were not be discernable just only the bands at ~1535 and 1502 cm1 in the NF-1 and NF-2
Fig. 3. ATR-FTIR spectra of PES-2, NF-1, and NF-2 membranes.
96
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
membranes. From the ATR-FTIR spectra (Fig. 3), no difference between these three membranes could be perceived. However, our experiment results showed that the PES-2, NF-1, and NF-2 membranes performances were different, especially in uranium rejection. These differences, to the best of our knowledge, are due to the proprietary layer structure. Understanding details of relationship between the material of the membranes and the membrane performance needs further study. In general, all the membranes had very similar spectra peaks. The similarity of spectra shows that these membranes have the same basic structure. Basic properties of three nanofiltration membranes that were supplied by the manufacturer are presented in Table 2. According to Table 2, the structural materials of the membranes are similar. Therefore, the obtained results from the ATR-FTIR analysis are reasonable. In another research (Singh et al., 2006), similar results were found for polyethersulfone and polyamide membranes. SEM pictures of the surface and the cross-section of the membranes are shown in Fig. 4. Comparison of the surface structures shows that the NF-1 membrane has a relatively rough surface
(surface pores). Moreover, layer structures of the membranes are distinguishable from the cross-section views and the skin layers of the nanofiltration membranes have thicknesses below 100 nm (Yaroshchuk et al., 2013). There is a correlation between surface roughness and membrane fouling (nanofiltration membrane with a rough surface is more prone to fouling than nanofiltration membrane with a smoother surface) (Van der Bruggena et al., 2008; Artug and Hapke, 2006). The NF-1 membrane has higher surface porosity than the PES-2 and NF-2 membranes but water permeability of this membrane is less than two other membranes (Fig. 2). Since the hydrophobic nanofiltration membranes have lower permeability, this finding may be ascribed to higher hydrophobic property of NF-1 membrane than the PES-2 and NF-2 membranes (Artug and Hapke, 2006). Therefore, fouling propensity of NF-1 membrane is more than two other membranes because of surface roughness and hydrophobicity. Membrane fouling may be reduced by applying membranes with lower surface roughness and higher hydrophilicity of the membrane surface (Van der Bruggena et al., 2008).
Fig. 4. SEM pictures of the (right) surface and (left) cross-section of PES-2, NF-1, and NF-2 membranes.
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
3.2. Effect of feed pH on membranes performance The solution pH is a very important factor which can change the nature of the membrane surface charge and pore size, as well as type and concentration of the dissolved metal species and consequently can influence the membrane separation performance (AlRashdi et al., 2013; Gherasim et al., 2013). Therefore, the solution pH is often the key parameter to control the desired separation (Van der Bruggena et al., 2008). Fig. 5 presents the effect of pH on the rejection of uranium and the permeate flux in three nanofiltration membranes at operating pressure of 10 bar and 15 mg/l uranium feed solution. It was found that the effects of feed pH on performance of polyethersulfone nanofiltration membrane (PES-2) and polyamide nanofiltration membranes (NF-1 and NF-2) were different. The results showed that the permeate flux of PES-2 membrane increased with an increase in the pH ranges of 4e6. Moreover, the rejection of uranium increased with increasing the pH from 4 to 6. Further increase of pH from 6 to 9 resulted in the decrease of
97
permeate flux and the rejection of uranium. This phenomenon could be attributed to the isoelectric point of PES-2 membrane at the pH 6 since the maximum permeate flux and rejection were observed around the isoelectric point of some nanofiltration membranes (Al-Rashdi et al., 2013; Shang et al., 2014). Therefore, the optimum pH for uranium removal by means of PES-2 nanofiltration membrane was determined 6. A similar trend was observed by Shang et al. (2014). They observed that permeate flux increased with an increase in the pH ranges of 2.5e6.5 while the rejection of vanadium remained constant. More increase of pH led to a decrease in the permeate flux and especially the rejection of vanadium. So, they reported that pH 6e6.5 is the optimum pH for concentrating vanadium from acid leach solution using DL and DK nanofiltration membranes. The other researchers explained that the pore size of the nanofiltration membranes changes at the isoelectric pH due to the expanding or shrinking of the skin layer of the membrane, the lowest electroviscous effect, and the highest net driving force in consequence of lowest osmotic pressure at the surface of the membrane (Al-Rashdi et al., 2013). For NF-1 and NF-2 membranes, the uranium rejection increased significantly within the pH ranges from 3 to 9. For both membranes the permeate flux first decreased and then remained constant for NF-1 while a little increase observed for NF-2 as the feed pH increased. Membrane charge density of negatively charged membrane also rises with increase of pH due to the dissociation of acidic groups which is reflected as an increase in salt rejection (Artug, 2007). Thus, these results indicated that NF-1 and NF-2 are negatively charged membranes. A speciation distribution was determined with the aid of the Visual Minteq software. The obtained results showed that the predicted species in a solution containing 16.2 mg/l UO2þ and 2 þ2 þ 7.44 mg/l NO 3 at pH ranges of 3e4 were UO2 and UO2OH . The þ complexes of (UO2)3(OH)þ 5 and (UO2)4(OH)7 dominated the system in pH ranges of 5e7, but at pH 9, (UO2)3(OH)-7 and UO2(OH)-3 were the major species. The more general feature of nanofiltration membranes is the separation of ions according to their size and valency (Favre-Reguillon et al., 2005). Thus, a high rejection coefficient of this bulky and highly charged uranium species can be expected. Briefly, small uranyl complexes were predominant within low pH ranges while the big complexes had been dominated by increasing the pH. Moreover, a big complex has a bigger hydrated radius than the small uranyl complexes. Therefore, higher rate of uranium was rejected at higher pHs. According to the obtained results, the PES-2 membrane had the highest permeate flux and the lowest uranium rejection because the PES-2 membrane was a loose nanofiltration membrane. Also, the uranium rejection by NF-1 membrane was higher than that of NF-2 membrane in the pH ranges of 3e4 but uranium rejection by NF-1 membrane was lower than that of NF-2 membrane with an increase in the pH ranges of 6e9. On the other hand, the NF-1 membrane had lower permeate flux than the NF-2 membrane in the studied pH range (3e9). This result confirmed that both of the charge exclusion and steric exclusion are important for uranium rejection in the NF-1 and NF-2 membranes. Anyway, surface structures of these two nanofiltration membranes were different (according to the SEM pictures), hence the surface charge behavior could be relatively different between these membranes with the same construction materials. 3.3. Effect of applied pressure on membranes performance
Fig. 5. Effect of pH on the rejection of uranium and the permeate flux of PES-2, NF-1, and NF-2 membranes at operating pressure 10 bar for 15 mg/l uranium feed solution.
Uranium rejection and permeate flux as a function of the operating pressure are shown in Fig. 6. These results were related to feed solutions with 15 mg/l uranium concentration at pH 6. For all
98
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
Fig. 6. Effect of operating pressure on the rejection of uranium and the permeate flux of PES-2, NF-1, and NF-2 membranes for feed solutions with 15 mg/l uranium concentrations at pH 6.
membranes the permeate flux increased with increasing the pressure from 5 to 20 bar while the uranium rejection by these membranes followed different trends. Generally, the pressure effect can be described by two mechanisms which control the uranium separation. First, an increase in the transmembrane pressure leads to an increase in the water flux but the uranyl ions transport is hindered by the steric and charge effects (the ion flux and water flux are uncoupled). Therefore, the higher water flux results in a dilution of permeate and consequently a higher rejection. Second, increasing the pressure will cause more uranyl ions transport to the membrane surface and, so the concentration polarization will increase which leads to a decrease in uranium rejection by decreasing the charge effect. The results of these two mechanisms that occur simultaneously will determine the rejection behavior at different transmembrane pressures (Gherasim et al., 2013; Fang and Deng, 2014). The overall effect of operating pressure on uranium rejection is controlled by these two competing mechanisms. The results of PES-2 membrane
indicated that the more ions transporting toward the membrane surface plays a dominant role. An increase in uranium rejection by NF-1 membrane as the pressures changes from 5 to 10 bar is generally explained by a shift in the transport mechanism across the membrane. At lower pressures a diffusive transport of uranium occurs which accounts for the lower rejections while convective transport of uranyl ions through the membrane is dominant mechanism at pressure 10 bar. When the applied pressure increases further, the uranium rejections decrease because the concentration polarization effects start to become significant. As mentioned before, concentration polarization which increases with increasing the pressure, decreases the rejection albeit the convective transport results in an increase in rejection. The counteracting contributions of the increased convective transport and the increased concentration polarization resulted in a nearly constant uranium rejection value for NF-2 membrane. Accordingly, in terms of uranium rejection, NF-2 had relatively stable behavior in spite of pressure changes and the behavior of NF-2 was different from the other two membranes. The increase of the permeate fluxes of all of these membranes with transmembrane pressure increase indicates that an increase in operating pressure will enhance the driving force and then will overcome the membrane resistance (Zuo et al., 2008). Also, as the pressure increases, convective transport and concentration polarization become more significant (Al-Rashdi et al., 2013). Fig. 6 shows liner change in the permeate flux with increasing the pressure. Although a similar and undeniable result was observed in the case of arsenic removal using a commercial nanofiltration membrane (NF270) in another research (Al-Rashdi et al., 2013), the increase in permeate flux of NF270 membrane was not linear for copper, lead and manganese with the rise of pressure which showed a quantity of concentration polarization. Moreover, the permeate fluxes at pressure 10 bar for PES-2, NF-1, and NF-2 membranes are 1698, 45, and 212 l/(hm2), respectively (Fig. 6). These results are lower than the corresponding pure water flux, 2314, 62, and 286 l/(hm2) for PES-2, NF-1, and NF-2, respectively (Fig. 2). This can be taken as an indication of membrane fouling and/or osmotic pressure build up caused by the retained salt. A similar reduction in the permeate flux was observed by Al-Rashdi et al. (2013). It should be noted that increase of the permeate flux of membrane leads the concentration polarization effects to become exponentially larger. In the laboratory systems, concentration polarization is controlled by increasing the bulk mixing of the feed solution. But, there are some practical limitations for this approach in industrial systems. Improving design of membrane module and developing methods of controlling the feed solution flow in the membrane module are the best solution for minimizing the effects of concentration polarization (Baker, 2004). Also, the concentration polarization could be quantified by the degree of concentration polarization which is defined as the ratio of the solute concentration at the surface of membrane and the solute concentration in the bulk of solution (Bi et al., 2014). In this research, it was tried to reduce the effects of this phenomenon by bulk mixing of the feed solution. The obtained results showed that there was a quantity of concentration polarization as it could be indicated by the nonlinear relationship between uranium rejection and pressure. 3.4. Effect of feed concentration on membranes performance Membranes performances in terms of the filtration flux and uranium rejection efficiency were studied by varying the feed uranium concentration from 7.5 to 238 mg/l at feed pH 6. The operating pressure was held constant at 10 bar. Fig. 7 shows the results of PES-2, NF-1, and NF-2 membranes. It was found that the
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
99
The rejection of uranium by polyamide nanofiltration membranes increased with an increase in initial concentration. The uranium rejection by NF-1 membrane changed from 57 to 79% as the initial uranium concentration increased from 7.5 to 238 mg/l. Similarly, the uranium rejection by NF-2 membrane increased from 62 to 98%. The increase of the uranium rejection with increase of initial uranium concentration suggested that the transport of uranium through the membrane did not increase proportional to the increase of uranium concentration. The increase of solute rejection with increment of feed concentration can be explained by the adsorption of the solute on the membrane surface (Al-Rashdi et al., 2013; Chung et al., 2005). There are two reasons for this claim. First, the compact membranes of NF-1 and NF-2 would be expected to adsorb higher amount of the solute on the membrane surface due to smaller pore size than the wide-open structure PES-2 membrane. Second, the operational pH for all experiments was adjusted at 6. As be noted before, isoelectric point of PES-2 membrane was 6 and thus the surface charge of this membrane was zero at pH 6. However, the NF-1 and NF-2 membranes had negative surface charge and the positive complexes of (UO2)3(OH)þ and 5 (UO2)4(OH)þ 7 were the dominant species of uranium at this pH. Therefore, on the basis of above discussions, adsorption of the uranium complexes on the surface of polyamide membranes is reasonable. It is interesting that the uranium rejection rate by NF-1 membrane was significantly lower than the NF-2 membrane. Probably the charge exclusion plays a critical role for uranium rejection at high initial uranium concentration rather than the steric exclusion. So, the NF-2 membrane has higher uranium rejection than the NF-1 membrane while NF-1 has the more compact structure than NF-2. It is obvious from Fig. 7 that the permeate flux of PES-2 is relatively constant but the permeate flux of polyamide nanofiltration membranes declines with increase of uranium concentration. These results can be described by two phenomena. First, adsorption or deposition of solute on the membrane surface leads to a decrease in the effective membrane pore size. Second, the increase of feed concentration leads to an increase in osmotic pressure (IAEA, 2004; Al-Rashdi et al., 2013). Accordingly, permeate flux decreased with increasing the feed concentration. 4. Conclusions Fig. 7. Effect of feed concentration on the rejection of uranium and the permeate flux of PES-2, NF-1, and NF-2 membranes at operating pressure 10 bar and pH 6.
effects of feed concentration on performance of polyethersulfone nanofiltration membrane (PES-2) and polyamide nanofiltration membranes (NF-1 and NF-2) were different. Because of the wide open structure, the rejection of PES-2 membrane was lower than two other membranes. As the feed uranium concentration increased, the uranium rejection of PES-2 decreased. The increased feed uranium concentrations (from 15 to 238 mg/l) caused about 90% decrease in uranium rejection efficiency. The downward trend was also reported for NF270 membrane as metal (Cd(II), Cu(II) and Pb(II)) concentrations increased from 100 to 2000 mg/l (Al-Rashdi et al., 2013). There are two phenomena by which increasing the concentration of ions on the membrane surface affect ions rejection. As the concentration increases, the electrostatic forces between the membrane and the ions in solution begin to weaken. Also, by increase of the concentration driving force for ions transport the diffusive factor of the transport through the membrane increased (IAEA, 2004; Chang et al., 2014). Both of these can lead to the decrease of uranium rejection.
Uranium removal from high-concentration aqueous solutions was studied by using three polyethersulfone (PES-2) and polyamide (NF-1 and NF-2) nanofiltration membranes under various operational conditions such as feed pH, transmembrane pressure, and uranium concentration of feed solution. It was found that the pH of the feed solution affects both uranium rejection and permeate flux because the charge properties of surface layer of the nanofiltration membranes and dissolved uranium species changed with pH. Increase of transmembrane pressure led to the permeate flux increase and the overall effect of operating pressure on the uranium rejection could be controlled by two competing mechanisms. Feed concentration affected the rejection and the permeate flux. For example, the uranium rejection by NF-1 and NF-2 membranes increased, whereas the permeate flux slightly decreased with an increase in feed solution concentration. The PES-2 membrane had higher permeate fluxes than the NF-1 and NF-2 membranes whereas uranium rejections by PES-2 membrane was lower than that of NF-1 and NF-2. Generally, the results of rejection and permeate flux showed that NF-2 polyamide membrane had the best performance compared to two other membranes. In previous researches, nanofiltration process was used for treatment of uranium-contaminated solutions with low concentration. But, wastewaters containing uranium in high concentration ranges are
100
M. Ghasemi Torkabad et al. / Progress in Nuclear Energy 94 (2017) 93e100
generated at some stages of the nuclear fuel cycle and there is the possibility of uranium release into the environment. Our research proves that the nanofiltration process, as a non-polluting separation method, can be effectively used for uranium removal and recovery from these solutions. Because, this method has desirable performance for uranium rejection and some unique advantages such as less energy consumption, modular structure, continuous performance, and ease of installation and automation over common technologies that used for treatment of radioactive wastewaters. Also, this method will provide some reductions including the amount of raw materials required, the effluent produced, the size of plant, and the overall related cost. References Al-Rashdi, B.A.M., Johnson, D.J., Hilal, N., 2013. Removal of heavy metal ions by nanofiltration. Desalination 315, 2e17. Anirudhan, T.S., Radhakrishnan, P.G., 2009. Improved performance of a biomaterialbased cation exchanger for the adsorption of uranium(VI) from water and nuclear industry wastewater. J. Environ. Radioact. 100, 250e257. Application of Membrane Technologies for Liquid Radioactive Waste Processing, 2004. IAEA, Vienna, p. 145. Artug, G., 2007. Modelling and Simulation of Nanofiltration Membranes. Cuvillier €ttingen, p. 248. Verlag, Go Artug, G., Hapke, J., 2006. Characterization of nanofiltration membranes by their morphology, charge and filtration performance parameters. Desalination 200, 178e180. Baker, R.W., 2004. Membrane Technology and Applications. John Wiley & Sons Ltd, p. 538. Bi, F., Zhao, H., Zhang, L., Ye, Q., Chen, H., Gao, C., 2014. Discussion on calculation of maximum water recovery in nanofiltration system. Desalination 332, 142e146. Chang, F., Liu, W., Wang, X., 2014. Comparison of polyamide nanofiltration and lowpressure reverse osmosis membranes on As(III) rejection under various operational conditions. Desalination 334, 10e16. Chellam, S., Clifford, D.A., 2002. Physicalechemical treatment of groundwater contaminated by leachate from surface disposal of uranium tailings. J. Environ. Eng. 128, 942e952. Chung, C.V., Buu, N.Q., Chau, N.H., 2005. Influence of surface charge and solution pH on the performance characteristics of a nanofiltration membrane. Sci. Technol. Adv. Mater. 6, 246e250. Diallo, H., Rabiller-Baudry, M., Khaless, K., Chaufer, B., 2013. On the electrostatic interactions in the transfer mechanisms of iron during nanofiltration in high concentrated phosphoric acid. J. Membr. Sci. 427, 37e47. Edwards, C.R., Oliver, A.J., 2000. Uranium processing: a review of current methods and technology. JOM 52, 12e20. Fang, J., Deng, B., 2014. Rejection and modeling of arsenate by nanofiltration: contributions of convection, diffusion and electromigration to arsenic transport. J. Membr. Sci. 453, 42e51. Favre-Reguillon, A., Lebuzit, G., Foos, J., Guy, A., Draye, M., Lemaire, M., 2003. Selective concentration of uranium from seawater by nanofiltration. Ind. Eng. Chem. Res. 42, 5900e5904. Favre-Reguillon, A., Lebuzit, G., Foos, J., Guy, A., Sorin, A., Lemaire, M., Draye, M., 2005. Selective rejection of dissolved uranium carbonate from seawater using
cross-flow filtration technology. Sep. Sci. Technol. 40, 623e631. Favre-Reguillon, A., Lebuzit, G., Murat, D., Foos, J., Mansour, C., Draye, M., 2008. Selective removal of dissolved uranium in drinking water by nanofiltration. Water Res. 42, 1160e1166. sek, P., 2013. Analysis of lead(II) retention from Gherasim, C.V., Cuhorka, J., Mikula single salt and binary aqueous solutions by apolyamide nanofiltration membrane: experimental results and modelling. J. Membr. Sci. 436, 132e144. Treatment of Liquid Effluent from Uranium Mines and Mills, 2004. IAEA, pp. 27e44. Jansson-Charrier, M., Guibal, E., Roussy, J., Surjous, R., Le Cloirec, P., 1996. Dynamic removal of uranium by chitosan: influence of operating parameters. Water Sci. Technol. 34, 169e177. Mishra, S.L., Vijayalakshmi, R., Singh, H., 2005. Extraction of uranium from acidic media with a novel synergistic mixture of di-nonyl phenyl phosphoric acid and tri-n-octyl phosphine oxide. Indian J. Chem. Technol. 12, 708e712. Misra, S.K., Mahatele, A.K., Tripathi, S.C., Dakshinamoorthy, A., 2009. Studies on the simultaneous removal of dissolved DBP and TBP as well as uranyl ions from aqueous solutions by using Micellar-Enhanced Ultrafiltration Technique. Hydrometallurgy 96, 47e51. Nouh, E.S.A., Amin, M., Gouda, M., Abd-Elmagid, A., 2015. Extraction of uranium(VI) from sulfate leach liquor after iron removal using manganese oxide coated zeolite. J. Environ. Chem. Eng. 3, 523e528. Raff, O., Wilken, R.D., 1999. Removal of dissolved uranium by nanofiltration. Desalination 122, 147e150. Roach, J.D., Zapien, J.H., 2009. Inorganic ligand-modified, colloid-enhanced ultrafiltration: a novel method for removing uranium from aqueous solution. Water Res. 43, 4751e4759. €fer, A.I., 2010. Impact of speciation on behaviour Rossiter, H.M.A., Graham, M.C., Scha of uranium in a solar powered membrane system for treatment of brackish groundwater. Sep. Purif. Technol. 71, 89e96. Sancho, M., Arnal, J.M., Verdú, G., Lora, J., Villaescusa, J.I., 2006. Ultrafiltration and reverse osmosis performance in the treatment of radioimmunoassay liquid wastes. Desalination 201, 207e215. Satpati, S.K., Roy, S.B., Pal, S., Tewari, P.K., 2015. Development of methodology for separation and recovery of uranium from nuclear wastewater. BARC Newsl. 347, 23e29. Shang, G., Zhang, G., Gao, C., Fu, W., Zeng, L., 2014. A novel nanofiltration process for the recovery of vanadium from acid leach solution. Hydrometallurgy 142, 94e97. Singh, P.S., Joshi, S.V., Trivedi, J.J., Devmurari, C.V., Rao, A.P., Ghosh, P.K., 2006. Probing the structural variations of thin film composite RO membranes obtained by coating polyamide over polysulfone membranes of different pore dimensions. J. Membr. Sci. 278, 19e25. €m, M., 2004. Long-term acid resistance and Tanninen, J., Platt, S., Weis, A., Nystro selectivity of NF membranes in very acidic conditions. J. Membr. Sci. 240, 11e18. Uranium 2014: Resources, production and demand, 2014. OECD Nuclear Energy Agency, France, p. 504. Uranium Extraction Technology, 1993. IAEA, Vienna, p. 380. Van der Bruggena, B., Manttari, M., Nystrom, M., 2008. Drawbacks of applying nanofiltration and how to avoid them: a review. Sep. Purif. Technol. 63, 251e263. Yaroshchuk, A., Bruening, M.L., Bernal, E.E.L., 2013. Solution-Diffusion-ElectroMigration model and its uses for analysis of nanofiltration, pressure-retarded osmosis and forward osmosis in multi-ionicsolutions. J. Membr. Sci. 447, 463e476. Zuo, W., Zhang, G., Meng, Q., Zhang, H., 2008. Characteristics and application of multiple membrane process in plating wastewater reutilization. Desalination 222, 187e196.