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Evolution of a Polysulfone Nanofiltration Membrane following Ion Beam Irradiation Rama Chennamsetty and Isabel Escobar* Department of Chemical & EnVironmental Engineering, The UniVersity of Toledo, 2801 West Bancroft, Nitschke Hall 3048, Toledo, Ohio 43606 ReceiVed January 11, 2008 Ion beam irradiation was used to modify the surface of a sulfonated polysulfone water treatment membrane. A beam of 25 keV H+ ions with four irradiation fluences (1 × 1013, 5 × 1013, 1 × 1014, and 5 × 1014 ions/cm2) was used to study the effects of ion beam irradiation on chemical structure, surface morphology, microstructure, and performance. XPS and ATR-FTIR analyses were performed on the virgin and irradiated membranes in order to determine the changes to chemical structure incurred by ion beam irradiation. The results showed that some sulfonic and C-H bonds were broken and new C-S bonds were formed after irradiation. AFM analysis showed that the roughness of the membranes decreased after irradiation, and the decrease in surface roughness was proportional to the increase in irradiation fluence. An increase in flux after ion beam irradiation was also observed along with a smaller flux decline during operation. Flux was not a function of irradiation fluence. Hydrophobicity, pore size distribution, and membrane rejection efficiencies were not affected by ion beam irradiation. Overall, irradiation led to an improvement in membrane performance.
Introduction Ideal membranes would be able to maintain high throughput of a desired permeate with a high rejection efficiency. Unfortunately, these two parameters are mutually counteractive.1 The reason is that a high degree of rejection is normally only achievable using a membrane having small pores and inherently high hydraulic resistance (or low permeability). Fouling is another severe problem associated with water treatment membranes, which is the deposition of solute constituents onto the surface of the membrane. Fouling of membrane elements often causes a significant increase in hydraulic resistance and applied pressure drop, which increases operating cost and decreases the life of the membrane.2 Membrane fouling is controlled by the foulant characteristics, feedwater solution chemistry (pH, ionic strength, divalent cation concentration), membrane properties (surface charge, hydrophobicity, roughness), and hydrodynamic conditions (permeate flux, crossflow velocity).3 There are two types of fouling: biofouling and abiotic fouling. Biofouling results from the accumulation of microorganisms on the membrane, while abiotic fouling forms a cake layer consisting of rejected materials, mainly natural organic matter (NOM) on the membrane surface. Since nanofiltration membranes have small pores, surface fouling which is caused by foulant accumulation (deposition) on the membrane surface is the dominant fouling mechanism in nanofiltration. The severity of abiotic fouling is directly linked to the membrane’s surface roughness.4 A membrane with high roughness has distinct * Corresponding author. Tel:+419 530 8267. Fax: 419 530 8086. E-mail:
[email protected]. (1) Good, K.; Escobar, I.; Xu, X.; Coleman, M.; Ponting, M. Desalination 2002, 146, 259–264. (2) Escobar, I. C.; Hoek, E. M.; Gabelich, C. J.; DiGiano, F. A.; Le Gouellec, Y. A.; Berube, P.; Howe, K. J.; Allen, J.; Atasi, K. Z.; Benjamin, M. M.; Brandhuber, P. J.; Brant, J.; Chang, Y. J.; Chapman, M.; Childress, A.; Conlon, W. J.; Cooke, T. H.; Crossley, I. A.; Crozes, G. F.; Huck, P. M.; Kommineni, S. N.; Jacangelo, J. G.; Kaeimi, A. A.; Kim, J. H.; Lawler, D. F.; Li, Q. L.; Schiddeman, L. C.; Sethi, S.; Tobiason, J. E.; Tseng, T.; Veerapanemi, S.; Zander, A. K. J. Am. Water Works Assoc. 2005, 97, 79–89. (3) Qilin, L.; Elimelech, M. EnViron. Sci. Technol. 2004, 38, 4683–4693. (4) Vrijenhoek, E. M.; Hong, S.; Elimelech, M. J. Membr. Sci. 2001, 188, 115–128.
peaks and valleys. The valleys provide the path of least resistance; therefore, a majority of permeate is transported through the membrane via these valleys. However, during operation, the valleys easily become clogged, which initiates membrane fouling and leads to complete blockage of the membrane pores. Polysulfone is an important material in the field of polymeric membranes because of its mechanical, thermal, and chemical stabilities as well as its excellent film-forming properties. It is used in different configurations, such as flat sheets or hollow fibers, primarily for ultrafiltration and nanofiltration separation applications.5,6 Ion beam irradiation has long been recognized as an effective method for the synthesis and modification of diverse materials, including polymers.7–14 Ion beam irradiation is the bombardment of a substance with energetic ions. When the ions penetrate through the surface of a membrane, they may eliminate the tall peaks and deep valleys, resulting in an overall reduction in surface roughness.10 As the ions penetrate the membrane, they lose energy to their surroundings (membrane structure) by two main processes: interacting with target nuclei (nuclear stopping) and interacting with target electrons (electronic stopping11). Nuclear stopping energy losses arise from collisions between energetic particles and target nuclei. Atomic displacement occurs when the colliding (5) Bungay, P. M.; Lonsdale, H. K.; de Pinho, I. N. Synthetic Membranes: Science, Engineering and Applications; D. Riedel Publishing Co.: Dordrecht, 1983. (6) Schott, K. Handbook of Industrial Membranes; Elseveier: Oxford, 1997. (7) Davenas, J.; Xu, X. L.; Boiteux, G.; Sage, D. Nucl. Instr. Methods Phys. Res. 1989, 8&9, 754–763. (8) Xu, X. L.; Dolveck, J. Y.; Boiteux, G.; Escoubes, M.; Monchanin, M.; Dupin, J. P.; Davenas, J. Mater. Res. Soc. Symp. Proc. 1995a, 354, 351–356. (9) Xu, X. L.; Dolveck, J. Y.; Boiteux, G.; Escoubes, M.; Monchanin, M.; Dupin, J. P.; Davenas, J. J. Appl. Polym. Sci. 1995a, 55, 99–107. (10) Xu, X. L.; Coleman, M. R. J. Appl. Polym. Sci. 1997, 66, 459–469. (11) Lee, E. H. Nucl. Instrum. Methods Phys. Res. Sect. B 1999, 151, 29–41. (12) Xu, X.; Coleman, M. R. Mater. Res. Soc. Symp. Proc. 1999a, 540, 255– 260. (13) Xu, X.; Coleman, M. R. Nucl. Instrum. Methods Phys. Res. Sect. B 1999, 152, 325–334. (14) Xu, X. L.; Coleman, M. R.; Myler, U.; Simpson, P. J. Post-Synthesis Method For DeVelopment of Membranes Using Ion Beam Irradiation of Polimide Thin Films; Pinnau, I. , Freeman, B. D. , Eds.; Oxford University Press: Washington, DC, 2000; pp 205-227.
10.1021/la8001042 CCC: $40.75 2008 American Chemical Society Published on Web 04/26/2008
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Figure 1. Structure of sulfonated polysulfone.
ion imparts energy greater than certain displacement threshold energy. If the energy is not great enough for displacement, the energy dissipates as atomic vibrations known as phonons. The threshold energy is the energy that a recoil requires to overcome binding forces and to move more than one atomic spacing away from its original site. The interaction of an ion with a target nucleus is treated as the scattering of two screened particles, since the nuclear collision occurs between two atoms with electrons around protons and neutrons. Nuclear stopping varies with ion velocity as well as the charges of two colliding atoms, as it is derived with consideration of the momentum transfer from ion to target atom and the interatomic potential between two atoms. Electronic stopping energy losses arise from electromagnetic interaction between the positively charged ions and the target electrons. There are two mechanism; one is called glancing collision (inelastic scattering, distant resonant collisions with small momentum transfer), which is quite frequent but each collision involves a small energy loss (100 eV). Both collisions transfer energy in two ways: electronic excitation and ionization. Excitation is the process in which an electron jumps to a higher energy level, while in ionization an orbital electron is ejected from the atom. The goal of the study described here was to determine the effects of ion beam irradiation on surface morphology, microstructure, and chemical structure and also the changes in the performance of modified commercial sulfonated polysulfone nanofiltration water treatment membranes.
Experimental Section Membrane Properties. The polysulfone membrane was a commercially available nanofiltration composite membrane manufactured by Hydranautics (San Diego, CA) with a selective layer of sulfonated polysulfone. The structure of the sulfonated polysulfone is shown in Figure 1. The membrane is negatively charged (at pH ) 7.0, the charge is -6.26 mV),15 hydrophobic (contact angle of 58°;16 Table 3), and has a molecular weight cutoff (MWCO) of 1000 Da.17 The operating temperature range is 0-45 °C, pH range is 2-11, and the membrane is able to withstand chlorine concentrations of several hundred ppm. Contact Angle Analysis. The contact angle measurements were performed using distilled water on the top surface of the membranes using a Cam-Plus Micro contact angle meter (Tantec Inc., Schaumburg, IL). The sessile drop, half-angle measuring method (U.S. Patent No. 5,268,733) was used for measuring the contact angles. The measurements were performed at room temperature. Pore Size Distribution Analysis. The pore size distribution analyses were performed by observing the rejections of various molecular weights (ranging from 200 to 4600 g/mol) of polyethylene glycol (PEG). The rejections were calculated by using the Phoenix 8000 total organic carbon (TOC) analyzer (Tekmar-Dohrmann, Cincinnati, OH). Before analysis, samples were filtered through a (15) Peng, W.; Escobar, I. C. EnViron. Prog. 2005, 392, 399. (16) Ma¨ntta¨ri, M.; Puro, L.; Nuortila-Jokinen, J.; Nystro¨m, M. J. Membr. Sci. 2000, 165, 1–17. (17) Bargeman, G.; Vollenbroek, J. M.; Straatsma, J.; Schroe¨n, C. G. P. H.; Boom, R. M. J. Membr. Sci. 2005, 247, 11–20.
Chennamsetty and Escobar 0.45 µm pore size nylon filter (Whatman International Ltd., Maidstone, England) to remove any solids and prevent damage to the analyzer. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) was used to study the evolution of chemical structures of the nanofiltration membranes after irradiation with a H+ ion beam. XPS is a quantitative spectroscopic technique used to identify the changes in the elemental composition of the membranes after irradiation. XPS analyses were done on a Kratos Axis Ultra X-ray photoelectron spectrometer using monochromated unmonochromated Al Ka excitation (hV ) 1486.6 eV) operating at 14 kV and 10 mA at the University of Michigan (Ann Arbor, MI). The pressure in the vacuum chamber was less than 10-9 mbar during acquisition. A takeoff angle of 90 ° was used for all analyses. Membranes samples were mounted on a sample holder with the use of an adhesive tape and kept overnight at high vacuum in the preparation chamber before they were transferred to the analysis chamber of the spectrometer. Each spectral region was scanned until a good signal-to-noise ratio was observed. Survey scan spectra, from which the overall atomic composition was computed, were acquired in 300 s using a pass energy of 160 eV and a 0.5 eV step. Fourier Transform Infrared Spectroscopy (FTIR) Analysis. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to study the evolution of chemical structures of the nanofiltration membranes after irradiation with a H+ ion beam. FTIR uses measurements of vibrational spectra to identify the chemical structure of materials. FTIR measurements were performed using a Digilab UMA 600 FT-IT microscope with a Pike HATR adapter and an Excalibur FTS 400 spectrometer (Randolph, MA). The FTIR analyses of the virgin and irradiated membranes with different fluences were performed under the same conditions before and after ion irradiation to monitor any changes induced by irradiation. The area under the peak was calculated for the irradiated samples for each peak of interest and normalized by the virgin area. This allowed for comparisons of slightly different areas. Scanning Electron Microscopy (SEM). SEM images were obtained using a Philips XL30FEG scanning electron microscope to examine surface structure and morphology changes of the membranes induced by irradiation. Specifically, SEM was used to visually quantify the level of abiotic fouling that occurred after filtration experiments of the fouled membrane surfaces and cross section. SEM analyses were also conducted to obtain cross section images of the membranes to determine the thickness of the top selective layer, which was required to calculate the energy for ion beam irradiation to modify only the membrane top selective layer. For cross section analysis, samples were soaked in water for several minutes and placed into liquid nitrogen. The membranes were then snapped and a crisp edge was produced. The samples were coated with a thin layer of gold under an argon atmosphere and placed in the scanning electron microscope for analysis. The membranes must be gold-coated because they are transparent to the electron beam used by the SEM. If the membrane is not finely covered with an electron-opaque substance like gold, the electron beam would travel right through the membrane, creating no image and probably destroying the membrane sample in the process. Atomic Force Microscope (AFM) Analysis. AFM was used to monitor the evolution of surface morphology. AFM allows acquiring 3D topographic data with a high vertical resolution. Accurate and quantitative data about surface morphology are provided over a wide range of magnifications and can be used in several quantitative analyses approaches, such as section, bearing, and roughness analysis. AFM measurements were performed using a Nanoscope IIIa scanning probe microscope (Digital Instruments, Santa Barbara, CA) in tapping mode. The membrane sample is mounted on a piezoelectric tube that moves the sample in the z direction to maintain a constant force, and in the x and y directions for scanning the sample. The resulting map of the area s ) f(x,y) represents the topography of the sample. A peak is defined as a point that has a height greater than the defined plane (x, y). The software directly provides the values for the mean roughness and the number of peaks in defined area of the membrane.
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The software was equipped with a surface roughness determination and peak counting function, which allowed the investigation of the impact of ion beam irradiation on the surface of the membrane. Ra, the mean value of the surface relative to the center plane, was calculated using eq 1
Ra )
1 LxLy
∫0L ∫0L |f(x, y)| dx dy x
y
(1)
where f(x,y) was defined as the surface relative to the center plane and Lx and Ly were the dimensions of the surface. Bench-Scale Cross-Flow. The membrane was housed in a SEPA CF cross-flow filtration unit (Osmonics, Minneatonka, MN). The filtration unit was constructed out of 316 stainless steel and rated for an operating pressure up to 69 bar (1000 psi). The test unit was sealed by applying adequate pressure via a hand pump (P-142, Enerpac, Milwaukee, WI), which actuated a piston on the SEPA CF, sealing the membrane within the membrane cell. The feed stream was delivered by a motor (Baldor Electric Co., Ft. Smith, AR, and Dayton Electric Manufacturing Co., Niles, IL) and M-03 Hydracell pump (Wanner Engineering, Inc., Minneapolis, MN) assembly. Flow valves controlled permeate and retenate (also called concentrate) flow, as well as the pressure acting on the membrane in the test unit. Due to the high pressures required by the membranes, it was necessary to control the temperature of the feedwater using a chiller (Model KR60A, Cole-Parmer Instrument Co., Veenon Hills, IL) to keep it at approximately 20 °C. The SEPA CF unit required a membrane area of 139 cm2, but membrane size was restricted to a circular area of 95 cm2 because of limitations of the chosen irradiation method. The required area was reduced to accommodate the sample size by applying a foil tape masking around the membrane (necessary also for irradiation purposes). The shear rate (i.e., height of the membrane channel) was equal to the feed spacer thickness, 0.7 mm. The membranes were tested with two synthetic feedwater solutions: (1) organics (humic acid, 17 mg/L; peptone, 10.5 mg/L; beef, 7.5 mg/L) with 0.5 mM CaCl2 and 8.5 mM of NaCl (total ionic strength of 10 mM) and (2) bovine serum albumin (BSA) protein at a concentration of 20 mg/L with 0.5 mM CaCl2 and 8.5 mM of NaCl (total ionic strength of 10 mM). Before testing was initiated, each membrane was thoroughly rinsed with deionized (DI) water and soaked in DI water overnight.18 The membrane was then removed from the DI water and thoroughly rinsed with DI water again immediately before placing the membrane into the filtration cell unit. After installation, the membrane was precompacted by filtering 250 mL of DI water. The presoaking and initial DI water flush were required to stabilize the flux and rejection of the membrane. Initially, membrane pores were filled with air; soaking and precompaction forces out these air bubbles, allowing a steady-state operation of the membrane prior to filtration of raw water. The synthetic raw water was added to the feed reservoir, and permeate samples were collected at regular intervals of time. Flux values were calculated by measuring the amount of permeate collected for a unit amount of time using a graduate cylinder. The operating conditions (i.e., initial flux, cross-flow velocity, and temperature) at this stage were identical to those applied during the initial precompaction with DI water, so as to determine the flux decline by comparing the fluxes determined at the starting of the filtration with feedwater for the virgin membrane to the flux of the membranes after 10 h of operation. The ratio of the flux at 20 °C of the membranes after 10 h of operation with feedwater, Jf, to the initial flux of virgin membrane, Jo,
flux decline (%) )
()
Jf × 100 Jo
(2)
was named the flux decline and used to evaluate the changes of the membrane flux after irradiation. Additional rejection analyses were performed by measuring the rejections of monovalent cations (NaCl, 10 mmol/L), divalent ions (18) Hong, S.; Elimelech, M. J. Membr. Sci. 1997, 132, 159–181.
(CaCl2, 10 mmol/L), and Orange II dye (molecular weight of 350 g/mol with a concentration of 10 mg/L). The concentrations of the salts were analyzed by a traceable expanded range digital conductivity meter (Fischer Scientific) and the Orange II dye was analyzed using a UV spectrophotometer (UV-2401 PC, Shimadzu Scientific Instruments, Addison, IL) at 486 nm wavelength. Rejection was calculated by using the equation
( )
rejection (%) ) 1 -
cp × 100 cf
(3)
where cp and cf are the solute concentrations of permeate and feedwater, respectively. Cleaning Experiments. Membrane resilience was determined by fully cleaning used membranes with disodium ethylenediaminetetraacetate (Na2-EDTA) and NaOH. The cleaning analyses were performed using two commonly used cleaning agents. Thus, cleaning efficiencies would be realistic to applications. Cleaning experiments were conducted using both the virgin and irradiated membranes to determine the change in irreversible fouling after ion beam irradiation. The effect of ion fluence on cleaning was also determined by conducting membrane cleaning experiments after irradiation at different ion fluences. Metal chelating agents, such as EDTA, remove divalent cations from the complexed organic molecules and improve the cleaning of the fouled membrane.18 There are five categories of cleaning agents: alkaline solutions, acids, metal-chelating agents, surfactants, and enzymes. Commercial cleaning products are often mixtures of these compounds. EDTA was chosen for cleaning because other studies have shown that it provides better cleaning when compared to other cleaning agents.19 The pH of the EDTA solution was adjusted to 11 using 1.0 M NaOH. At pH 11.0, all the carboxylic functional groups of EDTA were deprotonated (pKa values are 1.99, 2.67, 6.16, and 10.26).20 The increase in the chelating ability of EDTA at pH 11.0 resulted in a more effective ligand-exchange reaction between EDTA and alginate-calcium complexes within the alginate gel layer. Consequently, the alginate gel layer was broken down more easily compared to lower pH and, thus, resulted in a higher cleaning efficiency. The membrane was first compacted with DI water until the permeate flux became constant, followed by the addition of feedwater. Fouling experiments were conducted with the two synthetic feedwater solutions described previously and were carried out for 8 h. After 8 h of operation, the solution in the feed reservoir was disposed and cleaning of the fouled membrane was performed. The concentration of EDTA used was 1 mM and it was adjusted to pH 11 using 1.0 M NaOH. Cleaning experiments with NaOH were also conducted at pH 11. At the end of the cleaning stage, the chemical cleaning solution in the reservoir was emptied, and the reservoir and the membrane cell were rinsed with DI water to flush out the residual chemical cleaning solution. Finally, the cleaned membrane was subjected to the second baseline performance with DI water to determine the pure water flux. The operating conditions (i.e., initial flux, cross-flow velocity, and temperature) at this stage were identical to those applied during the initial precompaction with DI water, so as to determine the cleaning efficiency by comparing the pure water fluxes determined before fouling and after cleaning. The ratio of the flux at 20 °C of fouled/cleaned membrane, Jc, to the initial flux of virgin membrane, Jo,
cleaning efficiency (%) )
() Jc Jo
× 100
(4)
DIwater
was named the cleaning efficiency and used to evaluate the recovery of the membrane flux. Ion Beam Irradiation. Each membrane was irradiated with H+ ions at an energy level of 25 keV. The incident energy of 25 keV was determined using a program titled “The Stopping and Range (19) Ang, W. S.; Lee, S.; Elimelech, M. J. Membr. Sci. 2006, 272, 198–210. (20) Budavari, S. (Ed.) The Merck Index, An Encyclopaedia of Chemicals, Drugs, and Biologicals; Merck and Co. Inc. Publishers: Rahway, NJ, 1989.
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Chennamsetty and Escobar Table 1. Time Required for Irradiation at Various Ion Fluences ion fluence (ions/cm2)
time of irradiation (min)
1 × 10 5 × 1013 1 × 1014 5 × 1014
3.6 17.8 35.6 177.8
13
Figure 2. The passage of an energetic ion in a polymer during ion beam irradiation with a total ion path length R, which gives a projected range, RP, along the direction parallel to that of the incident ion.
of Ions in Matter” (SRIM21) to ensure that the entire upper semipermeable sulfonated polysulfone layer was modified. When an energetic ion penetrates a polymer, it undergoes a series of collisions with the atoms and electrons in the target matrix. In these collisions, the implanted ion loses its energy by means of both the nuclear and the electronic interactions with the polymer, depending on the energy, atomic number of the ion, as well as the polymer material. The nuclear interactions consist of individual elastic collisions between the ion and target atom nuclei, whereas the electronic interactions can be viewed more as a continuous viscous drag phenomenon between the injected ion and the sea of electrons surrounding the target nuclei. The passage of an energetic ion in a polymer is illustrated in Figure 2. As shown in the figure, the ion does not travel in a straight path to its resting place due to collisions with target atoms. The range, R, is the total distance that the ion travels before coming to rest. However, in many applications of energetic ions in surface modification, it is not the total distance R traveled by the ion that is of interest but the projection of R normal to the surface (i.e., the penetration depth or the projected range RP). The mean projected range (RP), increases with ion energy and decreases with atomic number of the ion. The main parameters governing the penetration depth are the energy and atomic number of the implanted ion. A composite membrane is characterized by a thin semipermeable selective layer (3000-4000 Å thick approximately), supported on a porous substrate (about ∼50 µm thick) that, in turn, is bonded to a fibrous layer, which provides mechanical strength to the top layers without adding significant hydrodynamic resistance.22 Using scanning electron miscroscopy (SEM), the thickness of the membrane selective layer was determined from cross section images. Figure 3 shows SEM images of the cross-section of the virgin polysulfone membrane. The cross-section images of the membranes show the selective, porous substrate and fibrous layers present and also give approximate values for the selective layer of the members. The projected ranges were determined using SRIM to ensure that the entire upper semipermeable selective layer was modified. The inputs for the SRIM were the structures of the membranes, their densities, and the type of ions used to irradiate. SRIM is a group of programs that calculates the stopping and range of ions into matter using a quantum mechanical treatment of ion-atom collisions (assuming a moving atom as an “ion” and all target atoms as “atoms”).21 This calculation uses statistical algorithms that allow the ion to jump between calculated collisions and then average the collision results over the intervening gap. Figures 4–6 are connected. In Figure 5, the integration of the inverse of dE/dX with respect to the ion energy gives the penetration distance as plotted in Figure 4. In Figure 6, the surface area under the plot of the total energy losses must be close to 25 keV. The three figures were obtained directly (21) Ziegler, J. F.; Biersack, J. P.; Littmark, U. (Eds.) The Stopping and Range of Ions in Solids; Pergamon Press: New York, 1985; Vol. 1. (22) Belfer, S.; Purinson, Y.; Kedem, O. Acta Polym. 1998, 49, 574–582.
from the SRIM software. All three figures are connected and are shown in different ways for different purposes. From Figure 4, the amount of energy required to modify only the surface of the membrane could be determined directly. Figure 5 shows the electronic and nuclear stopping energy losses at various ion energies for the polysulfone membrane. Figure 6 shows the electronic and nuclear energy losses for the polysulfone membrane at the ion energy of 25 keV. Figure 4 shows the penetration depth or the projected ranges of the ions at various ion fluences for the polysulfone membrane. From the projectile ranges, an incident energy of 25 keV was chosen to ensure that the entire upper semipermeable selective layer was modified and to minimize penetration into porous substrate by the ion beam irradiation for both membranes. By keeping irradiation only to the upper semipermeable membrane, the impact of irradiation on the chemical, structural, morphological, and transport properties of the polymer could be compared. Four irradiation fluences (1 × 1013, 5 × 1013, 1 × 1014, and 5 × 14 10 ions/cm2) were used for ion beam irradiation of the membranes. The beam current density was maintained at low levels (
