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Removal of nuclides and boron from highly saline radioactive wastewater by direct contact membrane distillation
Desalination 394 (2016) 101–107
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Desalination journal homepage: www.elsevier.com/locate/desal
Removal of nuclides and boron from highly saline radioactive wastewater by direct contact membrane distillation Xia Wen a, Fuzhi Li a, Xuan Zhao a,b,⁎ a b
Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China Beijing Key Laboratory of Radioactive Wastes Treatment, Tsinghua University, Beijing 100084, PR China
H I G H L I G H T S • DCMD removed nuclides efficiently even when feed containing high salt and boron. • The rejection ratio of B kept higher than 99.97%, even when feed B was 5000 mg L− 1. • The co-precipitation of B with CaSO4 resulted in the increase of permeate B.
a r t i c l e
i n f o
Article history: Received 16 February 2016 Received in revised form 23 April 2016 Accepted 1 May 2016 Available online xxxx Keywords: Membrane distillation Low level radioactive wastewaters Nuclides Boron
a b s t r a c t The volume of low level radioactive wastewaters (LLRWs) produced from nuclear power plants (NPPs) should be concentrated to as small as possible before the final solidification. The direct contact membrane distillation (DCMD) method was used to treat highly saline LLRWs with commercial polypropylene (PP) membranes. The effect of different salt concentration on the rejections of nuclides (Co(II), Sr(II), Cs(I)) and boron (B) in DCMD was investigated in both short- and long-term concentration tests. Due to the hydrophobic properties of PP membrane, DCMD processes could give a high rejection for nuclides with DF above 105, which was weakly influenced by the concentrations of feed salt and B in LLRWs. B concentrations in the permeate remained below 2.0 mg L−1, with a rejection factor (RF) greater than 99.97%, even when feed B concentration reached 5000 mg L−1 or feed salt was maintained at 300 g L−1. Scaling of the DCMD system by CaSO4 resulted in a significant reduction in permeate flux. The addition of boric acid and high concentrations of NaNO3 minimized scaling by CaSO4 on the membrane surface. Scaling could result in an increase in permeate B concentrations due to the co-precipitation of B and CaSO4 on the membrane surface. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Membrane distillation (MD) involves evaporating water through pores of hydrophobic macroporous membranes. Since concentration polarization has a negligible impact on permeate flux, MD can be potentially applied to effectively treat highly saline brines [1–4]. The permeate flux of vacuum membrane distillation (VMD) would decrease by only 24% from the initial flux when the salt concentration of reverse osmosis (RO) seawater brine was concentrated to 300 g L−1 [1]. In addition, high rejection of non-volatile salt can be achieved, as the surface tension of the hydrophobic macroporous membrane prevents the passage of liquid [5–8]. The low level radioactive wastewaters (LLRWs) produced from the nuclear power plants (NPPs) must be concentrated to the largest ⁎ Corresponding author at: Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China. E-mail address: [email protected] (X. Zhao).
http://dx.doi.org/10.1016/j.desal.2016.05.001 0011-9164/© 2016 Elsevier B.V. All rights reserved.
feasible extent in order to minimize the costs of final solidification and long-term storage. In the NPPs with pressurized water reactor (PWR), LLRWs usually contains radioactive isotopes with the activity concentration of 104–106 Bq L−1, non-radioactive dissolved salt (e.g. borates, ), and organic complex builders (EDTA, citnitrate, Ca2+, Mg2+, PO3− 4 rate and oxalate) or surfactants [9–11]. Thus, the concentrations of radio-nuclides are far below 1 μg L−1 and even below 1 ng L−1. However, non-radioactive ions exist in LLRWs at concentrations are higher than 1 mg L− 1 and even up to several moles per liter in some cases [12,13], especially boron (B) in LLRWs, which is released from primary coolant in pressurized or boiling water reactors during the outage period, could amount to average concentration of 500 mg L−1 [14], far higher than the discharge standard of 2.0 mg L−1 in China. Therefore, any related technology must be effectively used to separate both radionuclides and B from LLRWs. At the same time, concentrates which contain the majority of radio-nuclides, B, and salts released during nuclear processes must be minimized in volume for long-term storage after solidification by cementation. Evaporation is traditionally used in the
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nuclear industry and offers high removal efficiency; however, this method suffers from high energy and costly equipment requirements. Ion exchange processes are also widely used for the treatment of LLRWs. Some commercial and special synthetic resins are particularly effective in the removal of B, e.g., glycidyl methacrylate–methyl methacrylate–divinyl benzene resin functionalized with iminodipropylene glycol has been shown to exhibit a B adsorption capacity of 32 mg g−1 [15]. However, due to the lack of regeneration in the nuclear industry, large amounts of residual radioactive resins are inevitably generated. These organic compounds present serious problems due to low radiation stability during long-term storage. RO processes can also remove nuclides effectively, however only provide a low removal efficiencies of B with the range of 40–80%. Though adjusting the solution pH to 9–10 has been shown to improve the removal efficiency of B to above 90%, large amounts of alkali are generally required for pH adjustment due to buffering of boric acid, which could in turn increase the salinity of the feed solution and lead to a decrease in the permeate flux of the RO membrane [16]. Moreover, increasing pH also tends to weaken the solubility of sparingly soluble salts (e.g., CaCO3), resulting in an increase of membrane scaling [17]. As an alternative, the MD process can provide high rejection, low energy consumption, and reduced equipment costs for the treatment of LLRWs. Most studies only focused on the effects of operation condition, salt concentration or membrane characters on the permeate flux and nuclides' rejection in the treatment of LLRWs by MD [18–20], but the investigation about the removal of high concentration of B and the effect of sparingly soluble salts in MD process from LLRWs were limited. In addition, several studies investigated the treatment of seawater or other industry wastewater by MD and found that the scaling of sparingly soluble salts caused serious permeate flux reduction [21,22], but the effect of scaling on the removal of B and nuclides from LLWRs by MD was seldom studied. Therefore, in this study, the influence of inorganic salt (NaNO3, CaSO4) concentration on the rejection of typical nuclides (e.g., cobalt (Co(II)), strontium (Sr(II)), and cesium (Cs(I))) and B in boron-containing LLRWs was investigated using a commercial PP hydrophobic membrane. Correlations between membrane surface properties and flux declines were identified.
2.2. Experimental set-up Experiments were carried out in a laboratory-scale installation as shown in Fig. 1. Raw water was pumped from a 5 L feed tank equipped with a temperature-control system where water flowed across the MD module through the shell-side. The MD module was equipped with 16 segments of hollow fibers with a diameter of 30 mm and effective membrane length of 140 mm. The total effective area of membranes in a module was 70 cm2. The distillate flowed through the lumen side of fibers and was cooled by a heat exchanger. The instantaneous distillate flux was monitored continuously and recorded in real time on a computer. The feed and distillate were recirculated by peristaltic pumps. The temperatures were monitored by thermocouple. For all experiments, the mean feed temperature (Tf) was maintained at 70 °C and the permeate temperature (Tp) was 20 °C. The feed velocity (Vf) and permeate velocity (Vp) were 850 mL min−1 and 75 mL min−1, respectively. Commercial hydrophobic polypropylene (PP) hollow fiber membranes (Accurel Q3/2 pp, Membrana GmbH, Germany) were used in this study and relevant membrane properties are summarized in Table 2. 2.3. Analytical methods The concentration of B in the feed and permeate samples was determined by ICP-OES (iCAP 7000: Thermo Fisher Scientific, Waltham, MA, USA). Co(II), Sr(II), and Cs(I) concentrations in both feed and permeate solutions were analyzed by ICP-MS (iCAPQ: Thermo Fisher Scientific, Waltham, MA, USA). The pH of the feed solution (with boric acid) was measured using a pH meter (DDSJ-308A: Leici, Shanghai, China). The permeate flux (J) was calculated by: J¼
Δm A Δt ρ
ð1Þ
where Δm is the variation in the mass of distillate (kg); A is the effective area of the membranes in the module (m2); Δt is the elapsed time of the tests (h) and ρ is the density of distillate (1000 kg m−3). The rejection of nuclides was evaluated using a decontamination factor (DF):
2. Materials and methods DF ¼
2.1. Materials CsNO3, Sr(NO3)2 and Co(NO3)2·6H2O solutions made in deionized water were used to simulate raw water containing Co (II), Sr (II) and Cs (I) at 100 mg L−1. To investigate the influence of salt concentration on the rejection of trace nuclides and B by the DCMD, 2 h tests were conducted with raw water spiked with different salt solutions (0–300 g L−1 of NaNO3 or 0–5000 mg L−1 of B). In addition, five long term concentration experiments were conducted to investigate the impact of salt (NaNO3 and CaSO4) and B concentrations on scaling in DCMD processes with feed solutions (S/1–S/5, Table 1). Following the concentration tests, corresponding membrane samples (M/1–M/5) were dried in vacuum and reserved for further analysis. All chemicals used in this research were of analytical reagent grade.
Cf Cp
ð2Þ
where Cf is the concentration of nuclides in the feed solution and Cp is the concentration of nuclides in the distillate.
Table 1 The composition of feed solution in the five concentration tests. Feed solution
Composition Co(II), Sr(II), Cs(II) (mg L−1)
B (mg L−1)
S/1 S/2 S/3 S/4 S/5
100 100 100 100 100
2000 2000 2000 2000
CaSO4 (mg L−1)
NaNO3 (g L−1) 50
2000 2000 2000
50
Fig. 1. The schematic diagram of the DCMD experimental set-up. (1) Feed tank, (2) peristaltic pump, (3) thermometer, (4) MD module, (5) heat exchanger, (6) distillate tank, (7) distillate collector, (8) electronic balance.
X. Wen et al. / Desalination 394 (2016) 101–107
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where Cf is the concentration of B in the feed solution and Cp is the concentration of B in the distillate. The theoretical permeate flux (JTh) was calculated based on the relationship between the flux and salt concentration determined from short term experiments negating the effects of scaling resistance. This flux was compared with the experimental permeate flux (JEx) of long term tests to evaluate the effect of scaling at different salt concentrations.
DF values of nuclides (Co(II), Sr(II), Cs(I)) remained steady between 105 and 106. In the nuclear industry, radioactive wastewater is typically concentrated to salt concentrations ranging from 200 to 300 g L− 1 through evaporation before solidification by cementation. It is difficult or in some cases impossible for RO systems to operate at salt concentrations of above 100 g L−1. When salt was concentrated to 13 g L−1, the permeate flux of the RO system was found to decline by 56% compared with the flux of pure water [23]. These results indicate that DCMD is more effective for the treatment of radioactive wastewater with high salt concentration. Values of membrane flux and nuclides' rejection at different boron concentrations are shown in Fig. 3. An increase in B concentration in the feed solution did not result in significant impacts on permeate flux, with a permeate flux decreasing by less than 5% when the feed B concentration was as high as 5000 mg L−1. There was also no discernible change in membrane hydrophobicity, as indicated by a water contact angle of the membrane of 130.66° ± 1.03° regardless of B concentration. Moreover, DF values of nuclides were found to remain fairly high (105) across the concentration range. These results suggest that the DCMD process using PP membranes can produce water with low concentrations of nuclides even at feed B concentration as high as 5000 mg L−1.
2.4. Membrane characterization
3.2. Removal of B
Changes in membrane morphology over the course of experiments were investigated using scanning electron microscopy (SEM) (Nova 600: FEI, USA) and the extent of scaling on the membrane surface was investigated through SEM-EDS and XPS (ESCALAB250XI: Thermo Fisher, USA). The membrane hydrophobicity was measured using the contact angle of pure water droplets (OCA20: dataphysics, Germany).
As shown in Fig. 4, the B concentration in distillate increased when the feed B concentration was increased from 500 to 5000 mg L−1. However, B concentrations remained below 0.80 mg L−1, less than the standard of 2.0 mg L− 1. The pH of the feed solution varied from 6.47 to 4.73 at B concentrations ranging from 500 to 5000 mg L−1. Rejection of B was consistently above 99.97% over the concentration range and at low feed pH. These results are consistent with the results of Hou et al. and Boubakri et al. [7,24], in which DCMD processes provided greater than 99.8% B rejection over a feed B concentration range of 0– 750 mg L−1 and a feed pH range of 3–11. Consequently, the DCMD process may provide a superior treatment method over RO process over wide B concentration and pH ranges. The B removal efficiency for RO processes is generally in the range of 40–80% at neutral pH, and can be highly sensitive to solution pH [16]. When NaNO3 was added to the feed solution containing 2000 mg L−1 B to investigate the effect of salinity on B removal, the permeate flux was observed to decrease linearly as NaNO3 was increased from 0 to 300 g L−1. These results can be attributed to the combined influence variations in water partial pressure and fluid viscosity. As shown in Fig. 5b, feed solution containing elevated NaNO3 concentrations (0–
Table 2 The properties of membrane. Parameters
Values
Inner diameter (mm) Thickness (mm) Mean pore size (μm) Porosity (%) Water contact angle (θ), ° LEPw (KPa)
0.60 0.20 0.20 73 128.5 ± 2.5 260
The rejection of B was expressed as a rejection factor (RF): RF ¼ ð1−
CP Þ 100% Cf
ð3Þ
3. Results and discussion 3.1. Removal of nuclides by DCMD The salt concentration was adjusted by adding NaNO3 to the raw water. As shown in Fig. 2a, the permeate flux was found to decrease linearly upon an increase in salt concentration. However, even when salt concentration reached 300 g L−1, the permeate flux only declined by approximately 20% compared with the initial flux. The decrease in flux is mainly attributed to negative influence of salt content on the vapor pressure of the feed solution, which drives water vapor through the membrane pores. As shown in Fig. 2b, high salt concentration has no discernible influence on nuclides' rejection in the DCMD process. The
Fig. 2. The effect of NaNO3 concentration in feed solution on permeate flux (a) and nuclides' rejection (b).
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Fig. 3. Variation of permeate flux (a) and nuclides' rejection (b) as a function of the feed B concentration.
100 g L−1) had negligible impact on the removal of B in the DCMD process. These results are consistent with the findings of Boubakri et al. [7] who found that the addition of 0–40 g L−1 NaCl to feed solutions produced no discernible effect on the permeate B concentration. However, the permeate B concentration was found to increase from 0.3 mg L−1 to 1.2 mg L−1 when the feed NaNO3 concentration increased from 100 to 300 g L−1. This observation was accompanied with an increase in B in the distillate and a corresponding decline in membrane/water contact angle (Fig. 5b). Results from the XPS analysis (Table 3) also indicated that B accumulated on the membrane surface. In contrast to the rejection behavior for nuclides, the transmembrane transport of B differs from common non-volatile inorganic salts. Generally, a small proportion of B in the solution chemically binds with water vapor based through hydrogen bonding and is able to pass through the membrane pores. If the bond strength between B, water vapor and the membrane were equivalent, then re-adsorption of B may take place inside the membrane pores. Therefore, the adsorbed B on the membrane surface can be transported through membranes by an adsorption-desorption mechanism, similar to transport of humic acid (HA) through membranes [25]. 3.3. Membrane scaling Permeate flux data from experiments conducted with five feed solution (S/1 - S/5) are shown in Fig. 6. The results indicate that a reduction in permeate flux of less than 20% occurred with feed solutions S/1 and S/
Fig. 4. Variation of permeate B and B rejection as a function of the feed B concentration.
2, in agreement with the theoretical permeate flux (JTh) data calculated based on Fig. 3a and Fig. 5a, respectively. Therefore, the decline in permeate flux can be primarily attributed to the reduction in water vapor pressure on the two sides of the membrane caused by high boric acid and NaNO3 concentrations. However, when feed solutions contained CaSO4, the permeate flux deviated considerably from the theoretical flux. For feed solution S/3, membrane scaling was followed by a significant drop in permeate flux at ca. 11.5 h (t1) when the CaSO4 concentration in the feed tank was 3000 mg L−1. After 18.5 h of operation, the permeate flux declined by ca. 74% of the initial flux. Compared with S/3, the addition of boric acid in feed solution S/4 was observed to delay the decrease in permeate flux. Permeate flux declined by 52% of its initial flux after 19.0 h and with the addition of boric acid, the pH of the feed solution decreased to 4.92–5.05. The drop in pH mitigated deposition of CaSO4 on the membrane surface. For solution S/5, a 14% decline in permeate flux was observed after 26.4 h, which was consistent with the theoretical flux. After 26.4 h, the CaSO4 concentration was found to be 7900 mg L−1 and the permeate flux sharply declined to levels below the theoretical value due to CaSO4 scaling. This phenomenon is likely caused by an increase in CaSO4 solubility at high NaNO3 concentrations [17]. The coprecipitation of foulants is a common occurrence in membrane desalination systems and often contributes to scaling issues with membrane processes. The co-precipitation of CaSO4 and CaCO3 has been previously observed in MD process [26]. In addition, the coexistence of other ions, including magnesium, sodium, sulfate and HA can inhibit the precipitation of CaCO3 [27,28]. The variation in permeate B concentration and nuclides' rejection with the concentrate of feed solution are shown in Fig. 7. The permeate B concentration was found to increase with time. Especially, in experiments with S/4 and S/5, the permeate B concentration both increased sharply from t1 and t2 in conjunction with a decrease in permeate flux (Fig. 6). The XPS results revealed that co-deposition of B with CaSO4 occurred on membranes M/4 and M/5. There was 32.33 at.% B deposited on surface of M/4, level considerably higher than those observed with M/1 and M/2 (Table 3). SEM results also suggested that large amounts of pillared CaSO4 crystals, along with amorphous B crystals, deposited on the membrane surface (Fig. 8a–c). These results reveal that the scaling of B co-precipitated with CaSO4 on the membrane surface resulted in increasing transport of B through membrane pores. However, scaling by CaSO4 and B on the membrane surface produced an insignificant effect on the rejection of nuclides in experiments, with the DF values of nuclides consistently above 104 regardless of operating time (Fig. 7b–c). The calcium concentrations in a membrane cross-section determined by SEM-EDS line analysis suggest that CaSO4 scaling only occurred on the membrane surface (Fig. 8d) and that no wetting
X. Wen et al. / Desalination 394 (2016) 101–107
105
Fig. 5. Effect of NaNO3 concentration in feed solution on permeate flux (a) and rejection of B (b).
occurred along the membrane pore, allowing for high rejection of nuclides. 4. Conclusion The DCMD process using commercial PP membranes was found to be more effective compared with other membrane technologies for removal of nuclides and B from highly saline LLRWs. DCMD can help reduce the volume of radioactive brines and the costs associated with solidification and long-term storage. The high B and salt concentrations in feed solutions produced minimal impacts on the permeate flux and no marked effect on nuclides' rejection with DF values greater than 105. The permeate B concentration increased with an increase in B and salt concentrations, but remained consistently under the water quality standard of 2.0 mg L−1, even at B concentrations of 5000 mg L−1 and NaNO3 concentrations of 300 g L− 1. Scaling by CaSO4 resulted in a sharp decline in permeate flux when the feed CaSO4 concentration reached 3000 mg L−1. The presence of CaSO4 in feed solutions caused a sharp increase in permeate B concentrations due to co-precipitation Table 3 The XPS results of membranes after treating with different kinds of solution. Sample No.
Elements
Atom (%)
Original membrane
B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s
0.17 0.78 0.11 98.92 0.69 1.17 0.19 98.05 1.10 0.91 0.11 97.88 4.83 1.35 2.45 91.37 32.33 25.82 0.50 41.35 25.96 16.00 4.67 53.37
Membrane (H3BO3 + NaNO3)
Membrane (M/1)
Membrane (M/2)
Membrane (M/4)
Membrane (M/5)
of B with CaSO4 on the membrane surface. However, it exhibited no obvious influence on the rejection of nuclides and the DF values remained above 104 in all experiments. This was attributed to CaSO4 scaling being limited to the membrane surface with no penetration into the interior pores. The DCMD with high rejections of B and nuclides could offer a promising alternative to treat the high boron LLWs produced from NPPs. Acknowledgements This research was supported by State Administration of Science, Technology and Industry for National Defence and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13026). References [1] J. Mericq, S. Laborie, C. Cabassud, Vacuum membrane distillation of seawater reverse osmosis brines, Water Res. 44 (2010) 5260–5273. [2] X. Ji, E. Curcio, S. Al Obaidani, G. Di Profio, E. Fontananova, E. Drioli, Membrane distillation-crystallization of seawater reverse osmosis brines, Sep. Purif. Technol. 71 (2010) 76–82.
Fig. 6. Permeate flux as a function of elapsed time in long-term concentration tests.
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Fig. 7. The variation of permeate B and nuclides' rejection in long-term concentration tests as a function of elapsed time. Note: The DF values of Sr(II) in the tests with CaSO4 addition were not calculated since the co-precipitation of Sr(II) with CaSO4 in feed solution. [3] C.R. Martinetti, A.E. Childress, T.Y. Cath, High recovery of concentrated RO brines using forward osmosis and membrane distillation, J. Membr. Sci. 331 (2009) 31–39. [4] J. Minier-Matar, A. Hussain, A. Janson, F. Benyahia, S. Adham, Field evaluation of membrane distillation technologies for desalination of highly saline brines, Desalination 351 (2014) 101–108. [5] A. Criscuoli, P. Bafaro, E. Drioli, Vacuum membrane distillation for purifying waters containing arsenic, Desalination 323 (2013) 17–21. [6] D. Qu, J. Wang, D. Hou, Z. Luan, B. Fan, C. Zhao, Experimental study of arsenic removal by direct contact membrane distillation, J. Hazard. Mater. 163 (2009) 874–879. [7] A. Boubakri, S.A. Bouguecha, I. Dhaouadi, A. Hafiane, Effect of operating parameters on boron removal from seawater using membrane distillation process, Desalination 373 (2015) 86–93. [8] S. Yarlagadda, V.G. Gude, L.M. Camacho, S. Pinappu, S. Deng, Potable water recovery from As, U, and F contaminated ground waters by direct contact membrane distillation process, J. Hazard. Mater. 192 (2011) 1388–1394. [9] G. Pátzay, L. Weiser, F. Feil, G. Patek, Modification of the radioactive wastewater treatment technology in the Hungarian PWR, Desalination 321 (2013) 72–76. [10] X. Zhang, F. Li, X. Zhao, The investigation of radioactive wastewater from nuclear power plants, Nucl. Saf. 65-70 (2015) (In Chinese). [11] Y. Park, Y. Lee, W.S. Shin, S. Choi, Removal of cobalt, strontium and cesium from radioactive laundry wastewater by ammonium molybdophosphate–polyacrylonitrile (AMP–PAN), Chem. Eng. J. 162 (2010) 685–695. [12] D. Chen, X. Zhao, F. Li, Influence of boron on rejection of trace nuclides by reverse osmosis, Desalination 370 (2015) 72–78. [13] D. Chen, X. Zhao, F. Li, Treatment of low level radioactive wastewater by means of NF process, Nucl. Eng. Des. 278 (2014) 249–254. [14] F. Li, D. Sun, Main pollutants from inland AP1000 NPP liquid radioactive effluents and their treatment technologies, At. Energy Sci. Technol. 46 (2012) 137–141 (in Chinese with English abstract). [15] B.F. Senkal, N. Bicak, Polymer supported iminodipropylene glycol functions for removal of boron, React. Funct. Polym. 55 (2003) 27–33. [16] M.H. Oo, L. Song, Effect of pH and ionic strength on boron removal by RO membranes, Desalination 246 (2009) 605–612.
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Fig. 8. The SEM analysis of membrane surface of (a): M/3, (b): M/4, (c): M/5, and the SEM-EDS line analysis of cross section (d).
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Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Removal of nuclides and boron from highly saline radioactive wastewater by direct contact membrane distillation Xia Wen a, Fuzhi Li a, Xuan Zhao a,b,⁎ a b
Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China Beijing Key Laboratory of Radioactive Wastes Treatment, Tsinghua University, Beijing 100084, PR China
H I G H L I G H T S • DCMD removed nuclides efficiently even when feed containing high salt and boron. • The rejection ratio of B kept higher than 99.97%, even when feed B was 5000 mg L− 1. • The co-precipitation of B with CaSO4 resulted in the increase of permeate B.
a r t i c l e
i n f o
Article history: Received 16 February 2016 Received in revised form 23 April 2016 Accepted 1 May 2016 Available online xxxx Keywords: Membrane distillation Low level radioactive wastewaters Nuclides Boron
a b s t r a c t The volume of low level radioactive wastewaters (LLRWs) produced from nuclear power plants (NPPs) should be concentrated to as small as possible before the final solidification. The direct contact membrane distillation (DCMD) method was used to treat highly saline LLRWs with commercial polypropylene (PP) membranes. The effect of different salt concentration on the rejections of nuclides (Co(II), Sr(II), Cs(I)) and boron (B) in DCMD was investigated in both short- and long-term concentration tests. Due to the hydrophobic properties of PP membrane, DCMD processes could give a high rejection for nuclides with DF above 105, which was weakly influenced by the concentrations of feed salt and B in LLRWs. B concentrations in the permeate remained below 2.0 mg L−1, with a rejection factor (RF) greater than 99.97%, even when feed B concentration reached 5000 mg L−1 or feed salt was maintained at 300 g L−1. Scaling of the DCMD system by CaSO4 resulted in a significant reduction in permeate flux. The addition of boric acid and high concentrations of NaNO3 minimized scaling by CaSO4 on the membrane surface. Scaling could result in an increase in permeate B concentrations due to the co-precipitation of B and CaSO4 on the membrane surface. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Membrane distillation (MD) involves evaporating water through pores of hydrophobic macroporous membranes. Since concentration polarization has a negligible impact on permeate flux, MD can be potentially applied to effectively treat highly saline brines [1–4]. The permeate flux of vacuum membrane distillation (VMD) would decrease by only 24% from the initial flux when the salt concentration of reverse osmosis (RO) seawater brine was concentrated to 300 g L−1 [1]. In addition, high rejection of non-volatile salt can be achieved, as the surface tension of the hydrophobic macroporous membrane prevents the passage of liquid [5–8]. The low level radioactive wastewaters (LLRWs) produced from the nuclear power plants (NPPs) must be concentrated to the largest ⁎ Corresponding author at: Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China. E-mail address: [email protected] (X. Zhao).
http://dx.doi.org/10.1016/j.desal.2016.05.001 0011-9164/© 2016 Elsevier B.V. All rights reserved.
feasible extent in order to minimize the costs of final solidification and long-term storage. In the NPPs with pressurized water reactor (PWR), LLRWs usually contains radioactive isotopes with the activity concentration of 104–106 Bq L−1, non-radioactive dissolved salt (e.g. borates, ), and organic complex builders (EDTA, citnitrate, Ca2+, Mg2+, PO3− 4 rate and oxalate) or surfactants [9–11]. Thus, the concentrations of radio-nuclides are far below 1 μg L−1 and even below 1 ng L−1. However, non-radioactive ions exist in LLRWs at concentrations are higher than 1 mg L− 1 and even up to several moles per liter in some cases [12,13], especially boron (B) in LLRWs, which is released from primary coolant in pressurized or boiling water reactors during the outage period, could amount to average concentration of 500 mg L−1 [14], far higher than the discharge standard of 2.0 mg L−1 in China. Therefore, any related technology must be effectively used to separate both radionuclides and B from LLRWs. At the same time, concentrates which contain the majority of radio-nuclides, B, and salts released during nuclear processes must be minimized in volume for long-term storage after solidification by cementation. Evaporation is traditionally used in the
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nuclear industry and offers high removal efficiency; however, this method suffers from high energy and costly equipment requirements. Ion exchange processes are also widely used for the treatment of LLRWs. Some commercial and special synthetic resins are particularly effective in the removal of B, e.g., glycidyl methacrylate–methyl methacrylate–divinyl benzene resin functionalized with iminodipropylene glycol has been shown to exhibit a B adsorption capacity of 32 mg g−1 [15]. However, due to the lack of regeneration in the nuclear industry, large amounts of residual radioactive resins are inevitably generated. These organic compounds present serious problems due to low radiation stability during long-term storage. RO processes can also remove nuclides effectively, however only provide a low removal efficiencies of B with the range of 40–80%. Though adjusting the solution pH to 9–10 has been shown to improve the removal efficiency of B to above 90%, large amounts of alkali are generally required for pH adjustment due to buffering of boric acid, which could in turn increase the salinity of the feed solution and lead to a decrease in the permeate flux of the RO membrane [16]. Moreover, increasing pH also tends to weaken the solubility of sparingly soluble salts (e.g., CaCO3), resulting in an increase of membrane scaling [17]. As an alternative, the MD process can provide high rejection, low energy consumption, and reduced equipment costs for the treatment of LLRWs. Most studies only focused on the effects of operation condition, salt concentration or membrane characters on the permeate flux and nuclides' rejection in the treatment of LLRWs by MD [18–20], but the investigation about the removal of high concentration of B and the effect of sparingly soluble salts in MD process from LLRWs were limited. In addition, several studies investigated the treatment of seawater or other industry wastewater by MD and found that the scaling of sparingly soluble salts caused serious permeate flux reduction [21,22], but the effect of scaling on the removal of B and nuclides from LLWRs by MD was seldom studied. Therefore, in this study, the influence of inorganic salt (NaNO3, CaSO4) concentration on the rejection of typical nuclides (e.g., cobalt (Co(II)), strontium (Sr(II)), and cesium (Cs(I))) and B in boron-containing LLRWs was investigated using a commercial PP hydrophobic membrane. Correlations between membrane surface properties and flux declines were identified.
2.2. Experimental set-up Experiments were carried out in a laboratory-scale installation as shown in Fig. 1. Raw water was pumped from a 5 L feed tank equipped with a temperature-control system where water flowed across the MD module through the shell-side. The MD module was equipped with 16 segments of hollow fibers with a diameter of 30 mm and effective membrane length of 140 mm. The total effective area of membranes in a module was 70 cm2. The distillate flowed through the lumen side of fibers and was cooled by a heat exchanger. The instantaneous distillate flux was monitored continuously and recorded in real time on a computer. The feed and distillate were recirculated by peristaltic pumps. The temperatures were monitored by thermocouple. For all experiments, the mean feed temperature (Tf) was maintained at 70 °C and the permeate temperature (Tp) was 20 °C. The feed velocity (Vf) and permeate velocity (Vp) were 850 mL min−1 and 75 mL min−1, respectively. Commercial hydrophobic polypropylene (PP) hollow fiber membranes (Accurel Q3/2 pp, Membrana GmbH, Germany) were used in this study and relevant membrane properties are summarized in Table 2. 2.3. Analytical methods The concentration of B in the feed and permeate samples was determined by ICP-OES (iCAP 7000: Thermo Fisher Scientific, Waltham, MA, USA). Co(II), Sr(II), and Cs(I) concentrations in both feed and permeate solutions were analyzed by ICP-MS (iCAPQ: Thermo Fisher Scientific, Waltham, MA, USA). The pH of the feed solution (with boric acid) was measured using a pH meter (DDSJ-308A: Leici, Shanghai, China). The permeate flux (J) was calculated by: J¼
Δm A Δt ρ
ð1Þ
where Δm is the variation in the mass of distillate (kg); A is the effective area of the membranes in the module (m2); Δt is the elapsed time of the tests (h) and ρ is the density of distillate (1000 kg m−3). The rejection of nuclides was evaluated using a decontamination factor (DF):
2. Materials and methods DF ¼
2.1. Materials CsNO3, Sr(NO3)2 and Co(NO3)2·6H2O solutions made in deionized water were used to simulate raw water containing Co (II), Sr (II) and Cs (I) at 100 mg L−1. To investigate the influence of salt concentration on the rejection of trace nuclides and B by the DCMD, 2 h tests were conducted with raw water spiked with different salt solutions (0–300 g L−1 of NaNO3 or 0–5000 mg L−1 of B). In addition, five long term concentration experiments were conducted to investigate the impact of salt (NaNO3 and CaSO4) and B concentrations on scaling in DCMD processes with feed solutions (S/1–S/5, Table 1). Following the concentration tests, corresponding membrane samples (M/1–M/5) were dried in vacuum and reserved for further analysis. All chemicals used in this research were of analytical reagent grade.
Cf Cp
ð2Þ
where Cf is the concentration of nuclides in the feed solution and Cp is the concentration of nuclides in the distillate.
Table 1 The composition of feed solution in the five concentration tests. Feed solution
Composition Co(II), Sr(II), Cs(II) (mg L−1)
B (mg L−1)
S/1 S/2 S/3 S/4 S/5
100 100 100 100 100
2000 2000 2000 2000
CaSO4 (mg L−1)
NaNO3 (g L−1) 50
2000 2000 2000
50
Fig. 1. The schematic diagram of the DCMD experimental set-up. (1) Feed tank, (2) peristaltic pump, (3) thermometer, (4) MD module, (5) heat exchanger, (6) distillate tank, (7) distillate collector, (8) electronic balance.
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where Cf is the concentration of B in the feed solution and Cp is the concentration of B in the distillate. The theoretical permeate flux (JTh) was calculated based on the relationship between the flux and salt concentration determined from short term experiments negating the effects of scaling resistance. This flux was compared with the experimental permeate flux (JEx) of long term tests to evaluate the effect of scaling at different salt concentrations.
DF values of nuclides (Co(II), Sr(II), Cs(I)) remained steady between 105 and 106. In the nuclear industry, radioactive wastewater is typically concentrated to salt concentrations ranging from 200 to 300 g L− 1 through evaporation before solidification by cementation. It is difficult or in some cases impossible for RO systems to operate at salt concentrations of above 100 g L−1. When salt was concentrated to 13 g L−1, the permeate flux of the RO system was found to decline by 56% compared with the flux of pure water [23]. These results indicate that DCMD is more effective for the treatment of radioactive wastewater with high salt concentration. Values of membrane flux and nuclides' rejection at different boron concentrations are shown in Fig. 3. An increase in B concentration in the feed solution did not result in significant impacts on permeate flux, with a permeate flux decreasing by less than 5% when the feed B concentration was as high as 5000 mg L−1. There was also no discernible change in membrane hydrophobicity, as indicated by a water contact angle of the membrane of 130.66° ± 1.03° regardless of B concentration. Moreover, DF values of nuclides were found to remain fairly high (105) across the concentration range. These results suggest that the DCMD process using PP membranes can produce water with low concentrations of nuclides even at feed B concentration as high as 5000 mg L−1.
2.4. Membrane characterization
3.2. Removal of B
Changes in membrane morphology over the course of experiments were investigated using scanning electron microscopy (SEM) (Nova 600: FEI, USA) and the extent of scaling on the membrane surface was investigated through SEM-EDS and XPS (ESCALAB250XI: Thermo Fisher, USA). The membrane hydrophobicity was measured using the contact angle of pure water droplets (OCA20: dataphysics, Germany).
As shown in Fig. 4, the B concentration in distillate increased when the feed B concentration was increased from 500 to 5000 mg L−1. However, B concentrations remained below 0.80 mg L−1, less than the standard of 2.0 mg L− 1. The pH of the feed solution varied from 6.47 to 4.73 at B concentrations ranging from 500 to 5000 mg L−1. Rejection of B was consistently above 99.97% over the concentration range and at low feed pH. These results are consistent with the results of Hou et al. and Boubakri et al. [7,24], in which DCMD processes provided greater than 99.8% B rejection over a feed B concentration range of 0– 750 mg L−1 and a feed pH range of 3–11. Consequently, the DCMD process may provide a superior treatment method over RO process over wide B concentration and pH ranges. The B removal efficiency for RO processes is generally in the range of 40–80% at neutral pH, and can be highly sensitive to solution pH [16]. When NaNO3 was added to the feed solution containing 2000 mg L−1 B to investigate the effect of salinity on B removal, the permeate flux was observed to decrease linearly as NaNO3 was increased from 0 to 300 g L−1. These results can be attributed to the combined influence variations in water partial pressure and fluid viscosity. As shown in Fig. 5b, feed solution containing elevated NaNO3 concentrations (0–
Table 2 The properties of membrane. Parameters
Values
Inner diameter (mm) Thickness (mm) Mean pore size (μm) Porosity (%) Water contact angle (θ), ° LEPw (KPa)
0.60 0.20 0.20 73 128.5 ± 2.5 260
The rejection of B was expressed as a rejection factor (RF): RF ¼ ð1−
CP Þ 100% Cf
ð3Þ
3. Results and discussion 3.1. Removal of nuclides by DCMD The salt concentration was adjusted by adding NaNO3 to the raw water. As shown in Fig. 2a, the permeate flux was found to decrease linearly upon an increase in salt concentration. However, even when salt concentration reached 300 g L−1, the permeate flux only declined by approximately 20% compared with the initial flux. The decrease in flux is mainly attributed to negative influence of salt content on the vapor pressure of the feed solution, which drives water vapor through the membrane pores. As shown in Fig. 2b, high salt concentration has no discernible influence on nuclides' rejection in the DCMD process. The
Fig. 2. The effect of NaNO3 concentration in feed solution on permeate flux (a) and nuclides' rejection (b).
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Fig. 3. Variation of permeate flux (a) and nuclides' rejection (b) as a function of the feed B concentration.
100 g L−1) had negligible impact on the removal of B in the DCMD process. These results are consistent with the findings of Boubakri et al. [7] who found that the addition of 0–40 g L−1 NaCl to feed solutions produced no discernible effect on the permeate B concentration. However, the permeate B concentration was found to increase from 0.3 mg L−1 to 1.2 mg L−1 when the feed NaNO3 concentration increased from 100 to 300 g L−1. This observation was accompanied with an increase in B in the distillate and a corresponding decline in membrane/water contact angle (Fig. 5b). Results from the XPS analysis (Table 3) also indicated that B accumulated on the membrane surface. In contrast to the rejection behavior for nuclides, the transmembrane transport of B differs from common non-volatile inorganic salts. Generally, a small proportion of B in the solution chemically binds with water vapor based through hydrogen bonding and is able to pass through the membrane pores. If the bond strength between B, water vapor and the membrane were equivalent, then re-adsorption of B may take place inside the membrane pores. Therefore, the adsorbed B on the membrane surface can be transported through membranes by an adsorption-desorption mechanism, similar to transport of humic acid (HA) through membranes [25]. 3.3. Membrane scaling Permeate flux data from experiments conducted with five feed solution (S/1 - S/5) are shown in Fig. 6. The results indicate that a reduction in permeate flux of less than 20% occurred with feed solutions S/1 and S/
Fig. 4. Variation of permeate B and B rejection as a function of the feed B concentration.
2, in agreement with the theoretical permeate flux (JTh) data calculated based on Fig. 3a and Fig. 5a, respectively. Therefore, the decline in permeate flux can be primarily attributed to the reduction in water vapor pressure on the two sides of the membrane caused by high boric acid and NaNO3 concentrations. However, when feed solutions contained CaSO4, the permeate flux deviated considerably from the theoretical flux. For feed solution S/3, membrane scaling was followed by a significant drop in permeate flux at ca. 11.5 h (t1) when the CaSO4 concentration in the feed tank was 3000 mg L−1. After 18.5 h of operation, the permeate flux declined by ca. 74% of the initial flux. Compared with S/3, the addition of boric acid in feed solution S/4 was observed to delay the decrease in permeate flux. Permeate flux declined by 52% of its initial flux after 19.0 h and with the addition of boric acid, the pH of the feed solution decreased to 4.92–5.05. The drop in pH mitigated deposition of CaSO4 on the membrane surface. For solution S/5, a 14% decline in permeate flux was observed after 26.4 h, which was consistent with the theoretical flux. After 26.4 h, the CaSO4 concentration was found to be 7900 mg L−1 and the permeate flux sharply declined to levels below the theoretical value due to CaSO4 scaling. This phenomenon is likely caused by an increase in CaSO4 solubility at high NaNO3 concentrations [17]. The coprecipitation of foulants is a common occurrence in membrane desalination systems and often contributes to scaling issues with membrane processes. The co-precipitation of CaSO4 and CaCO3 has been previously observed in MD process [26]. In addition, the coexistence of other ions, including magnesium, sodium, sulfate and HA can inhibit the precipitation of CaCO3 [27,28]. The variation in permeate B concentration and nuclides' rejection with the concentrate of feed solution are shown in Fig. 7. The permeate B concentration was found to increase with time. Especially, in experiments with S/4 and S/5, the permeate B concentration both increased sharply from t1 and t2 in conjunction with a decrease in permeate flux (Fig. 6). The XPS results revealed that co-deposition of B with CaSO4 occurred on membranes M/4 and M/5. There was 32.33 at.% B deposited on surface of M/4, level considerably higher than those observed with M/1 and M/2 (Table 3). SEM results also suggested that large amounts of pillared CaSO4 crystals, along with amorphous B crystals, deposited on the membrane surface (Fig. 8a–c). These results reveal that the scaling of B co-precipitated with CaSO4 on the membrane surface resulted in increasing transport of B through membrane pores. However, scaling by CaSO4 and B on the membrane surface produced an insignificant effect on the rejection of nuclides in experiments, with the DF values of nuclides consistently above 104 regardless of operating time (Fig. 7b–c). The calcium concentrations in a membrane cross-section determined by SEM-EDS line analysis suggest that CaSO4 scaling only occurred on the membrane surface (Fig. 8d) and that no wetting
X. Wen et al. / Desalination 394 (2016) 101–107
105
Fig. 5. Effect of NaNO3 concentration in feed solution on permeate flux (a) and rejection of B (b).
occurred along the membrane pore, allowing for high rejection of nuclides. 4. Conclusion The DCMD process using commercial PP membranes was found to be more effective compared with other membrane technologies for removal of nuclides and B from highly saline LLRWs. DCMD can help reduce the volume of radioactive brines and the costs associated with solidification and long-term storage. The high B and salt concentrations in feed solutions produced minimal impacts on the permeate flux and no marked effect on nuclides' rejection with DF values greater than 105. The permeate B concentration increased with an increase in B and salt concentrations, but remained consistently under the water quality standard of 2.0 mg L−1, even at B concentrations of 5000 mg L−1 and NaNO3 concentrations of 300 g L− 1. Scaling by CaSO4 resulted in a sharp decline in permeate flux when the feed CaSO4 concentration reached 3000 mg L−1. The presence of CaSO4 in feed solutions caused a sharp increase in permeate B concentrations due to co-precipitation Table 3 The XPS results of membranes after treating with different kinds of solution. Sample No.
Elements
Atom (%)
Original membrane
B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s B1s Ca2p Na1s C1s
0.17 0.78 0.11 98.92 0.69 1.17 0.19 98.05 1.10 0.91 0.11 97.88 4.83 1.35 2.45 91.37 32.33 25.82 0.50 41.35 25.96 16.00 4.67 53.37
Membrane (H3BO3 + NaNO3)
Membrane (M/1)
Membrane (M/2)
Membrane (M/4)
Membrane (M/5)
of B with CaSO4 on the membrane surface. However, it exhibited no obvious influence on the rejection of nuclides and the DF values remained above 104 in all experiments. This was attributed to CaSO4 scaling being limited to the membrane surface with no penetration into the interior pores. The DCMD with high rejections of B and nuclides could offer a promising alternative to treat the high boron LLWs produced from NPPs. Acknowledgements This research was supported by State Administration of Science, Technology and Industry for National Defence and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13026). References [1] J. Mericq, S. Laborie, C. Cabassud, Vacuum membrane distillation of seawater reverse osmosis brines, Water Res. 44 (2010) 5260–5273. [2] X. Ji, E. Curcio, S. Al Obaidani, G. Di Profio, E. Fontananova, E. Drioli, Membrane distillation-crystallization of seawater reverse osmosis brines, Sep. Purif. Technol. 71 (2010) 76–82.
Fig. 6. Permeate flux as a function of elapsed time in long-term concentration tests.
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Fig. 7. The variation of permeate B and nuclides' rejection in long-term concentration tests as a function of elapsed time. Note: The DF values of Sr(II) in the tests with CaSO4 addition were not calculated since the co-precipitation of Sr(II) with CaSO4 in feed solution. [3] C.R. Martinetti, A.E. Childress, T.Y. Cath, High recovery of concentrated RO brines using forward osmosis and membrane distillation, J. Membr. Sci. 331 (2009) 31–39. [4] J. Minier-Matar, A. Hussain, A. Janson, F. Benyahia, S. Adham, Field evaluation of membrane distillation technologies for desalination of highly saline brines, Desalination 351 (2014) 101–108. [5] A. Criscuoli, P. Bafaro, E. Drioli, Vacuum membrane distillation for purifying waters containing arsenic, Desalination 323 (2013) 17–21. [6] D. Qu, J. Wang, D. Hou, Z. Luan, B. Fan, C. Zhao, Experimental study of arsenic removal by direct contact membrane distillation, J. Hazard. Mater. 163 (2009) 874–879. [7] A. Boubakri, S.A. Bouguecha, I. Dhaouadi, A. Hafiane, Effect of operating parameters on boron removal from seawater using membrane distillation process, Desalination 373 (2015) 86–93. [8] S. Yarlagadda, V.G. Gude, L.M. Camacho, S. Pinappu, S. Deng, Potable water recovery from As, U, and F contaminated ground waters by direct contact membrane distillation process, J. Hazard. Mater. 192 (2011) 1388–1394. [9] G. Pátzay, L. Weiser, F. Feil, G. Patek, Modification of the radioactive wastewater treatment technology in the Hungarian PWR, Desalination 321 (2013) 72–76. [10] X. Zhang, F. Li, X. Zhao, The investigation of radioactive wastewater from nuclear power plants, Nucl. Saf. 65-70 (2015) (In Chinese). [11] Y. Park, Y. Lee, W.S. Shin, S. Choi, Removal of cobalt, strontium and cesium from radioactive laundry wastewater by ammonium molybdophosphate–polyacrylonitrile (AMP–PAN), Chem. Eng. J. 162 (2010) 685–695. [12] D. Chen, X. Zhao, F. Li, Influence of boron on rejection of trace nuclides by reverse osmosis, Desalination 370 (2015) 72–78. [13] D. Chen, X. Zhao, F. Li, Treatment of low level radioactive wastewater by means of NF process, Nucl. Eng. Des. 278 (2014) 249–254. [14] F. Li, D. Sun, Main pollutants from inland AP1000 NPP liquid radioactive effluents and their treatment technologies, At. Energy Sci. Technol. 46 (2012) 137–141 (in Chinese with English abstract). [15] B.F. Senkal, N. Bicak, Polymer supported iminodipropylene glycol functions for removal of boron, React. Funct. Polym. 55 (2003) 27–33. [16] M.H. Oo, L. Song, Effect of pH and ionic strength on boron removal by RO membranes, Desalination 246 (2009) 605–612.
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Fig. 8. The SEM analysis of membrane surface of (a): M/3, (b): M/4, (c): M/5, and the SEM-EDS line analysis of cross section (d).
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