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Correlation Study between Free-Volume Holes and Molecular Separations of Composite Membranes for Reverse Osmosis Processes by Means of Variable-Energy Positron Annihilation Techniques Z. Chen,* K. Ito, H. Yanagishita, N. Oshima, R. Suzuki, and Y. Kobayashi National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan ABSTRACT: The nanoscopic structure of three kinds of commercially available composite membranes (LF10, NTR729HF, and NTR7250) for water purification has been studied by energyvariable positron annihilation. The membranes consist of a functional polymer film and porous polysulfone substrate supported on a nonwoven polyester fabric. Positron annihilation γ-ray and positron lifetime techniques were employed to probe the layer structure of the composite membranes and subnanometer-sized free-volume holes in the functional films, respectively. It was found that the functional film on the porous substrate in LF10 contains two different layers on the substrate, while that in NTR729HF and NTR7250 consists of a single top layer. Comparison of the rejections of different uncharged solutes, determined by a pressure-driven experiment, with the hole size of the functional film, obtained from the positron lifetime data, revealed that the holes probed by positrons can well explain the hindering effect of molecular transport. Moreover, the higher rejections by LF10 were found to arise from the denser top layer than the top layers of the other membranes. A combined use of the present positron techniques was demonstrated to be useful for examining the hole structure as well as the layer structure of the composite membranes.
’ INTRODUCTION Composite membranes have widely been applied to the reverse osmosis (RO) process for water purification and chemical waste treatment.1 Over the last decades, solute rejection and water flux of the composite membranes, characterizing their separation capability, have been substantially improved.2 However, for achieving even better separation and energy efficiencies, novel membranes that possess higher selectivity (solute rejection) and allow lower pressure operation of the RO process are required.3 At least two different components, that is, a thick supporting porous substrate with relatively large pores engineered as a path for water flow and a thin dense separation active film with several tens of nanometers thickness on the substrate, constitute the composite membrane for water purification.4 Of essential importance for the development of novel membranes with superior performance is a better understanding of the molecular transport mechanism in the separation active layer. Tsuru et al. developed a permeation model, based on the NernstPlanck equation,5 to characterize the separation membranes in terms of structural and electrical parameters.6 Bowen extended this model to include the steric effect and hindered transport of permeates through molecular-level holes in the separation active phase of the membranes.7 These studies show that the solute transport and the partitioning phenomena, governing the separation process, are influenced by the degree of solute confinement within the holes.6,7 For this reason, various techniques such as scanning electron microscopy,8 r 2011 American Chemical Society
atomic force microscopy,9 as well as positron annihilation1021 have been utilized to study the hole structure of the thin active layer of the composite membranes. To date, positron annihilation has been successfully applied to the characterization of various functional materials with engineered porosity including gas separation membranes,1113 pervaporation membranes,14,15 and nanofiltration membranes.16,17 Recent positron studies of the composite membranes for the RO process have suggested a possible connection between the hole structure probed by positrons and water flux.1621 In this work, positron annihilation based on energy-tunable positron beams is applied to the systematic examination of free-volume holes in composite membranes for water purification. The nanostructure of the membranes, probed by the positrons, is compared with the rejections that are determined by a conventional pressure-driven process using aqueous solutions of uncharged and charged compounds.
’ PRINCIPLE OF POSITRON ANNIHILATION Positron annihilation is well documented as a powerful tool for examining the hole structure of functional materials at the molecular level.22 Most frequently used in positron annihilation are positron annihilation lifetime and positron annihilation γ-ray techniques. The positron lifetime technique measures the Received: April 26, 2011 Revised: August 6, 2011 Published: August 11, 2011 18055
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lifetimes of positrons and positronium (Ps: the hydrogen-like bound state of a positron and an electron), whereas the positron annihilation γ-ray technique measures the Doppler broadening of annihilation radiation and the positron 3γ-annihilation probability. Some positrons implanted into insulators such as polymeric membranes may combine with one of the spur electrons to form Ps.23 In free space, spin-parallel triplet ortho-Ps (o-Ps) annihilates into 3γ rays with an intrinsic lifetime of 142 ns, while spinantiparallel singlet para-Ps (p-Ps) annihilates into 2γ-rays with a shorter lifetime of 125 ps. If o-Ps is localized in a molecularsized hole, it annihilates with a lifetime much shorter than 142 ns through a 2γ pick-off process upon collision with electrons on the hole walls. This quantum confinement of o-Ps in a smaller hole (especially of subnanometer size) reduces the probability of 3γ annihilation. The o-Ps lifetime by 2γ pickoff annihilation is well correlated with the hole dimension, so that the subnanometer-sized free volume can be examined by the positron annihilation lifetime technique.24,25 The relationship between the o-Ps lifetime τ3 [ns] and the hole radius r [nm] smaller than 1 nm has been confirmed by Nakanishi and Jean26,27 and Eldrup28 based on a simple quantum mechanical model of Tao29 as 1 r 1 2πr þ sin τ3 ¼ 0:5 1 r þ 0:166 2π r þ 0:166 ð1Þ The free-volume size Vf is calculated as 4 Vf ¼ πr 3 ð2Þ 3 When a positronelectron annihilation pair possesses a longitudinal momentum p, the annihilation γ rays are Doppler shifted from 511 keV by ( cp/2, where c is the speed of light.30 This brings about significant broadening of the 511 keV annihilation photo peak, which is evaluated as the line-shape S parameter defined as the relative counts of the central region in the 511 keV annihilation photo peak. The electrons involved in different chemical elements and bonds possess their own characteristic momentum distributions, so that the S parameter is sensitively influenced by the chemical structure of the molecules surrounding the positrons. In the presence of pores far larger than 1 nm, some o-Ps may be trapped therein before annihilation. Because of the reduced overlap of the Ps wave function with the electrons on the pore walls, such o-Ps hardly annihilates via the pick-off process and dominantly undergoes 3γ annihilation. Thus, the positron 3γ-annihilation probability provides useful information on porosity of various materials. Implementing depth selectivity to positron annihilation by combining it with variable-energy positron beams considerably broadens the applicability. Variation of incident positron energy over a wide range enables depth-profiling; meanwhile, tuning of the beam energy enables the study of surfaces, interfaces, and thin films.1420 The mean implantation depth of positrons zm [nm] is given by the following equation31 zm ¼
40 1:6 E F in
ð3Þ
where F is the material density in g/cm , and Ein is the positron incident energy in keV, respectively. For example, at 1 keV, zm is about 40 nm, and at 2 keV, it is about 120 nm for a material with a 3
density of unity. With the positron beam we are able to analyze the layer structure of the composite membranes from the variations of the positron annihilation characteristics as a function of Ein.
’ EXPERIMENTS Three commercially available membranes (LF10, NTR729HF, and NTR7250) were purchased from NITTO DENKO Corp., Japan. LF10 is a composite of functional films of polyamide and poly(vinyl alcohol) and porous polysulfone substrate supported on a nonwoven polyester fabric,32,33 and NTR729HF and NTR7250 are composites of polyamide-based film and the porous polysulfone substrate on the polyester fabric.4 The rejection of different charged and uncharged solutes by the membranes was determined at 25 °C through a conventional pressure-driven process based on the following equation Rj ¼ 1
Cp Cf
ð4Þ
where Rj is the rejection, and Cf and Cp are the concentrations of the solutes in the feed and permeate sides, respectively. Both the concentrations were evaluated by measuring the conductivity and the total amount of organic carbon for the charged salt ion and the neutral organic compounds, respectively. Rejections by the membrane samples with an effective area of 12.6 cm2 were measured at a feed flow rate of 50 cm3/min using 0.1 wt % aqueous solutions of the testing compounds under respective operation pressures of 1.0 MPa for NTR729HF and 1.5 MPa for LF10 and NTR7250. The solutions were stirred at 800 rpm to minimize the concentration polarization. Prior to each test, the membrane samples had been stabilized in pure water for 7 days under the prescribed pressures. Positron annihilation γ-ray spectra for the membranes were collected at different Ein with a 22Na-source-based magnetically guided positron beam system.34 The line-shape S parameter was determined as the ratio of the counts appearing in the central region (510.3511.7 keV) to the total counts of the 511 keV annihilation photo peak (506.8515.2 keV) in the spectrum recorded with a Ge detector. The 3γ annihilaltion of o-Ps leads to γ rays with energies lower than 511 keV. The positron 3γ-annihilation probability was characterized as the R parameter, which is determined as the ratio of the counts in a low-energy window (365.0495.0 keV) to those in the 511 keV annihilation photo peak (506.8515.2 keV) of the spectrum obtained as a function of Ein. An intense pulsed-positron beam generated with an electron linear accelerator35 was utilized for measuring lifetimes of positrons at different Ein. Lifetime data were recorded by determining the time interval between the timing signal from the pulsing system and the detection of an annihilation γ ray by using a BaF2 scintillation detector. The recorded positron lifetime data were analyzed assuming three or four exponential components plus background to deduce the average lifetime τ3 and relative intensity I3 for the long-lived component of o-Ps. The freevolume hole size was evaluated from τ3 using eqs 1 and 2.
’ RESULTS Four uncharged organic compounds of molecular weights similar to NaCl listed in Table 1 were chosen to compare rejections by the three composite membranes in the pressure-driven process. 18056
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Table 1. Molecular Weight and Density of the Uncharged Solutes for Rejection Test molecular
molecular uncharged
weight,aMw
density,aF
volume,bVm
molecule
[g/mol]
[g/cm3]
[nm3]
urea
60.06
1.320
0.076
ethylene glycol
62.07
1.113
0.093
1-propanol
60.1
0.803
0.124
2-propanol
60.1
0.786
0.127
a
The molecular weight and density were quoted from ref 36. b The volume of the uncharged molecule Vm is approximated as Vm = (Mw)/(NAF), where NA represents Avogadro’s constant.
Table 2. Rejection and Respective Operation Pressures for the Composite Membranes rejectiona [%] pressure membrane
(MPa)
ethylene
2-
NaCl urea
glycol
1-propanol
propanol
LF10
1.5
99.5
49.8
64.7
74.3
91.1
NTR729HF
1.0
74.6
7.8
18.2
18.3
36.5
NTR7250
1.5
62.3
6.1
15.5
14.1
23.9
Figure 1. Variations of the line-shape S parameter of the 511 keV positron annihilation photo peak for the composite membranes (LF10, NTR729HF, and NTR7250) as a function of positron incident energy Ein. A relative standard uncertainty due to the measurement repeatability was estimated as 0.15%. On the upper axis is shown mean implantation depth calculated from eq 3 with a density of 1 g/cm3. The lines are drawn for visual clarification only.
a
A relative standard uncertainty for each value due to the measurement repeatability was estimated as 5% at most.
These compounds have different chemical structures and molecular volumes. The molecular volume is smaller in the order of urea < ethylene glycol ∼2.2 keV, where R increases with Ein. During the thermalization process, energetic o-Ps travels over a few nanometers in a polymer matrix.37
Figure 2. Variations of the R parameter for the composite membranes as a function of Ein. The R parameter is defined as the ratio of 3γ annihilation counts in the lower-energy range (365.0495.0 keV) to the counts of the 511 keV 2γ annihilation peak (506.8515.2 keV). A relative standard uncertainty due to the measurement repeatability was estimated as 1.1% (comparable to the symbol size). On the upper axis is provided mean implantation depth as in Figure 1. The lines are drawn for visual clarification only.
If energetic o-Ps is formed at a depth from the surface shallower than the traveling distance, it may escape out from the surface, annihilating into 3γ rays in vacuum. In view of eq 3, increased R in the first stage (Ein < ∼0.8 keV) in comparison with the second stage (∼0.8 keV e Ein e ∼2.2 keV) can reasonably be attributed to escaping of energetic o-Ps into vacuum and its subsequent 3γ annihilation. In the second stage, where the positrons undergo dominantly 2γ annihilation in the dense top region of the composite 18057
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Figure 3. Variations of the ortho-positronium (o-Ps) lifetime τ3 for the composite membranes as a function of Ein. Free-volume hole radius obtained from τ3 using eq 1 is shown on the right-hand axis. A relative standard uncertainty due to the measurement repeatability and sample uniformity was estimated as 4.0% at most. On the upper axis is provided mean implantation depth as in Figure 1. The lines are drawn for visual clarification only.
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Figure 5. Correlation between S and I3 (SI3 plot) for the composite membranes measured at different Ein ranging from 0.8 to 5.0 keV. The arrows denote the directions of increasing Ein, and the values represent Ein for LF10.
membranes, τ3 first increases and then decreases with increasing Ein from 0.8 to ∼2.2 keV. At higher incident energies, it decreases toward a substrate value of ∼1.8 ns. The data of I3 are presented in Figure 4. For the LF10 membrane, I3 stays nearly constant in the range from 0.8 to 1.2 keV. With increasing Ein above 1.2 keV, it suddenly decreases to a minimum at Ein = 3.5 keV and then starts rising toward a value for the substrate at Ein = 5.0 keV. For NTR729HF and NTR7250, I3 gradually increases with increasing Ein over the studied range, indicating that the overall tendencies of I3 on Ein for these two membranes in Figure 4 are similar to the S parameters for the respective membranes in Figure 1.
’ DISCUSSION For Ps-forming materials like the present membranes, the overall S parameter can be related to the Ps formation probability f as S ¼ ð1 f ÞSeþ þ fSPs Figure 4. Variations of o-Ps intensity I3 for the composite membranes as a function of Ein. A relative standard uncertainty due to the measurement repeatability and sample uniformity was estimated as 9.1% at most. On the upper axis is provided mean implantation depth as in Figure 1. The lines are drawn for visual clarification only.
membranes, the lowest values of R are observed. In contrast, in the third stage (Ein > ∼2.2 keV), especially above 5.0 keV, R displays large values for all the membranes because a fraction of positrons undergo 3γ annihilation in the large pores of the substrate. Accordingly, we can relate the three stages of R to Ps annihilations (1) in vacuum/near surface, (2) in the dense film, and (3) in the porous substrate, respectively. Figure 3 shows an o-Ps lifetime τ3 on the left vertical axis and the free-volume hole radius evaluated from τ3 using eq 1 on the right vertical axis. As seen in Figure 3, τ3 of the LF10 membrane is ∼1.3 ns at Ein ranging from 0.8 to ∼2.2 keV. Above this energy range, it gradually increases and then approaches a value for the substrate (∼1.8 ns). For NTR729HF and NTR7250
ð5Þ
where Se+ and SPs represent the S parameters of free positrons and Ps, respectively.38 In the absence of spin conversion and chemical reaction, which is the case for the present membranes, one-fourth of Ps is p-Ps, and the rest is o-Ps. Hence SPs is expressed as 1 3 SPs ¼ Sp-Ps þ So-Ps 4 4
ð6Þ
where Sp-Ps and So-Ps are the S parameters for p-Ps and o-Ps, respectively. Since f is equal to 4/3 of I3, eq 5 can be rewritten in the following form 1 4 S ¼ Seþ þ I3 So-Ps þ Sp-Ps Seþ ð7Þ 3 3 From this equation, we expect a linear relation between S and I3 for a given system, provided that Se+, So-Ps, and Sp-Ps are constant. In this regard, eq 7 provides a vehicle to correlate our S data in Figure 1 and I3 data in Figure 4 with each other. Figure 5 shows plots of the S parameter vs I3, both recorded at various Ein ranging from 0.8 to 5.0 keV. For NTR729HF and 18058
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The Journal of Physical Chemistry C NTR7250, with increasing Ein the S parameters linearly increase with I3 from left to right, and the variations for the two membranes can be expressed as a single line. The identical linear correlation implies that Se+, So-Ps, and Sp-Ps are constant for both the membranes as a function of depth from the surface (eq 7). For LF10, in the energy range from 0.8 to 3.0 keV the S parameter decreases with decreasing I3 from right to left. At Ein = 3.0 keV it turns around and starts increasing with I3 up to Ein = 5.0 keV, following the same correlation as that for NTR729HF and NTR7250. The correlation between S and I3 for LF10 in the energy range from 0.8 to 3.0 keV has a distinctively smaller slope and a larger intercept at I3 = 0 than the corresponding correlation for the other membranes and LF10 with Ein above 3.0 keV. The larger intercept and smaller slope of the correlation can be justified by larger Se+. It has been reported38,39 that Se+ and hence the correlation between S and I3 are strongly dependent on the chemical structure of a polymer. Larger Se+ for LF10 at low positron energies may be attributed to the different chemical structure of the top layer from the other two membranes. As pointed out in previous papers,4,32,33 the top layer of NTR729HF and NTR7250 is of polyamide-based polymer,4 but that of LF10 contains poly(vinyl alcohol) in addition to polyamide.32,33 The smooth variations of S and I3 for NTR729HF and NTR7250 are obviously the result of the gradual increase of the mean implantation depth of the positrons with Ein. At low Ein, the positrons are mostly implanted into the top layer, exhibiting S and I3 characteristic for the top layer. With an increase in Ein, a fraction of the positrons are implanted into the porous substrate, S and I3 gradually approaching the values characteristic for the substrate. However, the V-shaped variations of S and I3 for LF10 (see Figures 1 and 4) clearly show that a second layer is present between the top layer and the substrate for this membrane. As we see from Figure 5, this second layer has similar Se+ to the top layer of NTR729HF and NTR7250, signifying that the layer is composed of chemical elements similar to those membranes. On the other hand, the top layer of LF10 has Se+ much different from polyamide and the porous substrate, supporting that this layer is composed of poly(vinyl alcohol). Desalination by the separation membrane is normally discussed in terms of the Donnan potential and the size exclusion effect. The previous discussion simply predicts that if the holes involved in the separation process are far smaller than a target solute the transport of the solute molecules should be effectively restrained. In the separation process, the holes in the separation membrane act as filtering channels,40 and the overall rejection of uncharged solutes can be expressed by taking account of the solute hindrance factor and the steric partitioning coefficient, both varying as a function of the size ratio of the solute to the hole.41 In the present study, we simply compare the rejection of different solutes by the three membranes with the ratio of the solute size Vm to the hole size Vf. The REin data in Figure 2 suggest that the active film can be detected by positrons in the range of ∼0.8 keV e Ein e ∼2.2 keV (the second stage). Only in this energy range is the lifetime free from Ps annihilations in vacuum/interface and in the porous substrate. So the freevolume hole radii in the active top layer for LF10, NTR729HF, and NTR7250 were evaluated from the averaged o-Ps lifetimes in the above energy range. Figure 6 shows a plot of the rejection versus the ratio of the solute size Vm to the hole size in the active top layer Vf. As one can
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Figure 6. Correlation between the rejection of the uncharged organic compounds and the ratio of the compound size to the free-volume hole size for the active layer in the three membranes. The hole size was evaluated from the average of τ3 obtained at Ein ranging from 0.8 to 2.0 keV. The tested compounds are urea, ethylene glycol, 1-propanol, and 2-propanol. The broken line is drawn for visual clarification only.
see, all the data fall on a single correlation, indicating that the evaluated size ratio of the molecules to the free-volume hole is closely associated with the molecular transport in the separation process for the present systems. In spite of the empirical nature of our approach, the universal correlation, valid not only for various solutes but also for different membranes, highlights the size exclusion nature of the rejection process using the studied membranes. As in the figure, when the ratio Vm/Vf is larger than unity, the corresponding rejection is higher than 10%. Therefore, the holes involved in the separation process are necessarily either comparable to or somewhat smaller than the solutes in size. In addition, LF10 with the smallest hole size in the active layer provides the highest rejections of all the compounds. According to the SI3 plot in Figure 5, the active top layer of LF10 contains poly(vinyl alcohol). This suggests that the smaller holes due to poly(vinyl alcohol) in addition to polyamide cause the higher rejection performance for LF10. The obtained results demonstrate that an important role in the molecular separation is indeed played by the hindering effect of the free-volume holes in the separation active layer.
’ CONCLUSION Variable-energy positron annihilation γ-ray and lifetime techniques have been applied to the characterization of three kinds of composite membranes for water purification. The overall Ein dependence of R can be divided roughly into the following three stages in terms of Ein, namely: (1) Ein < ∼0.8 keV, where R decreases with Ein, (2) ∼0.8 keV e Ein e ∼2.2 keV, where R exhibits a shallow minimum, and (3) Ein > ∼2.2 keV, where R increases with Ein. The three stages can be related to Ps annihilations in (1) vacuum/near surface, (2) the dense film, and (3) the porous substrate, respectively. The SI3 plots clearly demonstrated that LF10 consists of two different layers, that is, a poly(vinyl alcohol) top layer and a polyamide layer underneath the top layer, on the porous substrate, while NTR729HF and NTR7250 have a single active layer with chemical elements similar to the second layer of LF10. Comparison of the rejections of different uncharged organic solutes with the hole size obtained 18059
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The Journal of Physical Chemistry C from the o-Ps lifetimes in the second energy stage revealed that the free-volume holes can well explain the hindering effect of molecular transport. Moreover, the higher rejections by LF10 have been found to arise from the smaller holes in the functional films of poly(vinyl alcohol) and polyamide. A combined use of the present positron techniques was demonstrated to be useful for examining the hole structure as well as the layer structure of the composite membranes.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Prof. Y. Nagashima and his students of Tokyo University of Science are appreciated for the preparation of the positron moderator. Dr. Brian O’Rourke of AIST is appreciated for his valuable suggestions to the manuscript. This work was supported by NEDO. ’ REFERENCES (1) Wittmann, E.; Thorsen, T. In Nanofiltration: Principles and Applications; Schafer, A. I., Fane, A. G., Waite, T. D., Eds.; Elsevier: Amsterdam, 2005; pp 263265. (2) Linder, C.; Kedem, O. In Nanofiltration: Principles and Applications; Schafer, A. I., Fane, A. G., Waite, T. D., Eds.; Elsevier: Amsterdam, 2005; pp 726. (3) Manttari, M.; Kallioinen, M.; Pihlajamaki, A.; Nystrom, M. Water Sci. Technol. 2010, 62, 1653–1660. (4) Petersen, R. J. J. Membr. Sci. 1993, 83, 81–150. (5) Schultz, S. G. Basic Principles of Membrane Transport; Vail-Ballou: New York, 1980; Chapter 2. (6) Tsuru, T.; Urairi, M.; Nakao, S.; Kimura, S. J. Chem. Eng. Jpn. 1991, 24, 518–524. (7) Bowen, W. R.; Mohammad, A. W.; Hilal, N. J. Membr. Sci. 1997, 126, 91–105. (8) McCutcheon, J. R.; McGinnis, R. L.; Elimelech, E. J. Membr. Sci. 2006, 278, 114–123. (9) Hirose, M.; Ito, H.; Kamiyama, Y. J. Membr. Sci. 1996, 121, 209–215. (10) Shimazu, A.; Ikeda, K.; Miyazaki, T.; Ito, Y. Radiat. Phys. Chem. 2000, 58, 555–561. (11) Tanaka, K.; Katsube, M.; Okamoto, K.; Kita, H.; Sueoka, O.; Ito, Y. Bull. Chem. Soc. Jpn. 1992, 65, 1891–1897. (12) De Sitter, K.; Winberg, P.; D’Haen, J.; Dotremont, C.; Leysen, R.; Martens, J. A.; Mullers, S.; Maurer, F. H. J.; Vankelecom, I. F. J. J. Membr. Sci. 2006, 278, 83–91. (13) Merkel, T. C.; Freeman, B. D.; Spontak, R. J.; He, Z; Pinau, I.; Meakin, P.; Hill, A. J. Science 2002, 296, 519–522. (14) Li, B.; Liu, W. P.; Jiang, Z. Y.; Dong, X.; Wang, B. Y.; Zhong, Y. R. Langmuir 2009, 25 (13), 7368–7374. (15) Chen, H. M.; Hung, W. S.; Lo, C. H.; Huang, S. H.; Cheng, M. L.; Liu, G.; Lee, K. R.; Lai, J. Y.; Sun, Y. M.; Hu, C. C.; Suzuki, R.; Ohdaira, T.; Oshima, N.; Jean, Y. C. Macromolecules 2007, 40 (21), 7542–7557. (16) Cano-Odena, A.; Vandezande, P.; Hendrix, K.; Zaman, R.; Mostafa, K.; Egger, W.; Sperr, P.; De Baerdemaeker, J.; Vankelecom, I. F. J. J. Phys. Chem. B 2009, 113, 10170–10176. (17) De Baerdamaeker, J.; Boussu, K.; Djourelov, N.; Van der Bruggen, B.; Dauwe, C.; Weber, M.; Lynn, K. G. Phys. Status Solidi C 2007, 4 (10), 3804–3809. (18) Jean, Y. C.; Hung, W. S.; Lo, C. H.; Chen, H.; Liu, G.; Chakka, L.; Cheng, M. L.; Nanda, D.; Tung, K. L.; Huang, S. H.; Lee, K. R.; Lai, J. Y.; Sun, Y. M.; Hu, C. C.; Yu, C. C. Desalination 2008, 234, 89–98.
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