Environ. Sci. Technol. 2007, 41, 2008-2014
Effect of Flux (Transmembrane Pressure) and Membrane Properties on Fouling and Rejection of Reverse Osmosis and Nanofiltration Membranes Treating Perfluorooctane Sulfonate Containing Wastewater C H U Y A N G Y . T A N G , * ,† Q . S H I A N G F U , ‡ CRAIG S. CRIDDLE,† AND JAMES O. LECKIE† Environmental Engineering and Science, Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020
Perfluorooctane sulfonate (PFOS) is an emergent contaminant of substantial environmental concerns. In this study, reverse osmosis (RO) and nanofiltration (NF) membranes were used to remove this toxic and persistent compound from PFOS-containing wastewater. Five RO membranes and three NF membranes were tested at a feed concentration of 10 ppm PFOS over 4 days, and the PFOS rejection and permeate flux performances were systematically investigated. PFOS rejection was well correlated to sodium chloride rejection. The rejection efficiencies for the RO membranes were >99%, and those for the NF membranes ranged from 90-99%. Improvement in PFOS rejection, together with mild flux reduction (99%) has been demonstrated for four commercial RO membranes over a wide range of feed concentrations (11). To the authors’ best knowledge, the removal of PFOS by NF membranes has never been reported. As there are considerable differences between RO and NF membranes, it is worthwhile to compare the rejection efficiencies of the two types of membranes. One of the major challenges in the application of membrane technology is fouling -- significant flux loss may occur due to continuous accumulation of colloidal and organic matter, precipitation of inorganic salts, and/or microbial growth. While the feasibility of membrane treatment of PFOS has been demonstrated (11), the mechanism of flux loss (fouling) during PFOS removal has not been well understood, and the effect of membrane properties and hydrodynamic conditions on membrane fouling and PFOS rejection need to be investigated systematically. The objectives of this study were (a) to investigate the effect of membrane properties and hydrodynamic conditions on PFOS rejection and flux performance of both RO and NF membranes and (b) to understand the mechanism of flux reduction during PFOS removal. The flux and rejection performances of five RO and three NF membranes were evaluated with a crossflow test setup. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and streaming potential measurements were used to characterize PFOS fouled membranes. In addition, the amount of PFOS accumulated on fouled membranes was also determined by liquid chromatograph and tandem mass spectrometry (LC/ MS/MS). Such detailed characterizations enable in-depth understanding of the mechanisms of PFOS rejection and flux decline.
2. Materials and Methods 2.1. Chemicals and Materials. 2.1.1 General Chemicals. Unless otherwise specified, all reagents and chemicals were analytical grade with purity over 99%. The acid form of PFOS (100%) was supplied by AZ Electronic Materials USA Corp (Somerville, NJ) through SEMATECH (Austin, TX). MilliQ 10.1021/es062052f CCC: $37.00
2007 American Chemical Society Published on Web 02/15/2007
TABLE 1. Properties of RO and NF Membranes membrane
type
pure water flux (m/day)a
NaCl rejectiona (%)
rms roughness (nm)c
ζ potential at pH 4 (mV)
SG LFC1 LFC3 BW30 ESPA3 DK NF90 NF270
RO RO RO RO RO NF NF NF
0.77 ((0.06)b 1.31 ((0.06)d 0.93 ((0.06)b 1.31 ((0.10)b 2.49 ((0.12)b 1.83 3.72 ((0.12)d 5.00 ((0.25)d
95.2 ((0.5)b 97.3d 98.5 ((0.2)b 97.9 ((0.4)b 94.9 ((0.7)b 66.4 94.4 ((1.5)d 56.9 ((3.8)d
17.3 ((3.0) 135.8 ((12.8) 108.4 ((12.4) 68.3 ((12.5) 181.9 ((26.1) 16.4 ((3.1) 129.5 ((23.4) 9.0 ((4.2)
-12.3b 4.0d 3.8b 3.6b 14.7b 7.4 24.6d -5.6d
a Pure water flux was obtained after 2 days of membrane pre-compaction, and NaCl rejection determined as the ratio of permeate conductivity to feed conductivity (10 mM NaCl feed solution) at 24 h after adding NaCl to feed tank. A pressure of 1379 kPa (200 psi) at a temperature of 25 °C was used. Standard deviations are shown in parentheses. Refer to ref 11 for more details about the test conditions. b Reference 11. c Reference 15. d References 20 and 21.
water from a Millipore system (Billerica, MA) had a resistivity of 18.2 Mohm‚cm. Methanol (optima grade), isopropyl alcohol, sodium chloride, sodium hydroxide, and hydrochloric acid were purchased from Fisher Scientific (Santa Clara, CA). 2.1.2. RO/NF Membranes. Eight commercial thin film composite (TFC) polyamide membranes were used, including five RO membranes (BW30, ESPA3, LFC1, LFC3, and SG) and three NF membranes (DK, NF90, and NF270). DK and SG were obtained from GE Osmonics (Minnetonka, MN). ESPA3, LFC1, and LFC3 were provided by Hydranautics (Oceanside, CA), and BW30, NF90, and NF270 were supplied by Dow FilmTec (Minneapolis, MN). All membranes were stored as dry flat coupons in the dark. The virgin membrane properties (pure water flux, NaCl rejection, surface roughness, and ζ potential) are summarized in Table 1. 2.2. PFOS Quantification. PFOS concentrations in both feed and permeate samples were measured by LC/MS/MS according to Tang et al. (11) using a slightly modified method from Higgins et al. (10). A Sciex API 3000 triple quadropole mass spectrometer (MDS Sciex, Ontario, Canada) operating in negative electrospray ionization (ESI) multiple reaction monitoring (MRM) mode was employed (11). Briefly, samples were diluted by methanol to concentrations within the calibration range (0.1-25 ng/mL) prior to sample analysis. Two MRM transitions (primary transition, C8F17SO3- f FSO3-; secondary transition, C8F17SO3- f SO3-) with a dwell time of 100 ms were used for quantification and confirmation, respectively. The coefficient of determination (R2) was greater than 0.99 for standard curves generated from both the primary and secondary transitions, and the fitted values for all points were within 15% of their actual values (11). Higgins et al. (10) reported a recovery of 80-90% on the basis of spiking known amount of PFOS to sludge and sediment samples. The recovery in the current study is estimated to be 90-105%. 2.3. Membrane Performance Tests. PFOS rejection and membrane flux performance were evaluated by a customassembled setup consisting of CEPA CFII crossflow cells (GE Osmonics, Minnetonka, MN) arranged in parallel (11). Details of the testing procedures can be found in Tang et al. (11) and are briefly summarized here. Flat membrane coupons (14.6 cm × 9.5 cm) were thoroughly rinsed with MilliQ water, soaked in a MilliQ water bath for 24 h, and then precompacted for 48 h by filtering MilliQ water under desired operating pressure until a steady-state flux was obtained. Subsequently, PFOS were introduced to the feed tank, and the test was continued for another 15 min to 4 days before membrane coupons were taken out for further characterizations. Both the feed tank and permeate samples were collected for PFOS concentration determination, and membrane permeate flux was measured gravimetrically. A feed PFOS concentration of 10 ppm at pH 4.0 ( 0.1 and a crossflow at 1.37 L/min ((5%, corresponding to a crossflow velocity of 20 cm/s) were used for all tests in the current study. Such solution conditions
are representative of some semiconductor wastewaters (11). Unless otherwise specified, the transmembrane pressure was 1379 ( 7 kPa (200 ( 1 psi). 2.4. Membrane Characterization. PFOS-fouled membranes were characterized by AFM, XPS, and streaming potential measurements. In addition, the amount of PFOS accumulation on membranes was determined using LC/MS/ MS. Virgin membranes were rinsed with MilliQ water and soaked for 24 h before being used for XPS and streaming potential analysis. Fouled membranes were briefly rinsed in MilliQ water to remove labile PFOS molecules. They were air dried for AFM analysis and vacuum dried for XPS measurements. On the other hand, no drying was allowed for streaming potential analysis. 2.4.1. AFM. Tapping Mode AFM micrographs of fouled membranes were obtained with a MiltiMode SPM equipped with a J-type piezoelectric scanner and a NanoScope IV controller (Veeco, Santa Barbara, CA) according to Tang et al. (15). Single-crystal etched silicon probes (RTESP, Veeco, Santa Barbara, CA) were used, and a scan size of 5 by 5 µm2 was adopted. The Version 5.12 of the Nanoscope control software was used for image acquisition. 2.4.2. XPS. XPS is widely used by membrane researchers for membrane characterization and foulant analysis (1618). With a sampling depth less than 5 nm, XPS is highly surface sensitive (19). It is able to detect and quantify all atoms, except hydrogen, with a detection limit of ∼0.2 atomic percent. Elemental composition of clean and PFOS-fouled membranes were determined using an SSI S-Probe Monochromatized XPS Spectrometer with aluminum KR radiation as the X-ray source (1486 eV). Each spectrum was averaged from five scans sweeping over 0-1000 eV electron binding energy with a resolution of 1 eV. An electron neutralizer gun was operated at 3 eV to avoid sample charging. XPS depth profiling was also carried out to measure the fluorine signal in the top skin layers of fouled composite membranes. During the measurement, an argon ion gun were operated at approximately 500 V and 10 mA to etch away layers of membrane material followed by XPS elemental analysis at the etched depth. In this way, fluorine signal vs etching time plots could be obtained for PFOS-fouled membranes. The nominal rate of etching, calibrated against thermally grown SiO2 crystals, was 10.5 nm/min. The nominal etched depth was determined as the product of etching time and nominal etching rate. The actual depth of etching is expected to be greater than the corresponding nominal value, as organic-based polymeric material is typically etched faster than crystalline SiO2 (19). 2.4.3. Streaming Potential Measurements. Streaming potential measurements were performed for both virgin and fouled membranes using an electrokinetic analyzer equipped with an asymmetric clamping cell (Anton Paar, Graz, Austria), as described previously (11). A 10 mM NaCl background VOL. 41, NO. 6, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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electrolyte and a nitrogen headspace were maintained for all the tests. 2.4.4. LC/MS/MS for Determining PFOS Adsorption. PFOS-fouled membrane samples were soaked in 50% (by volume) isopropyl alcohol aqueous solution overnight on an orbital shaker to recover PFOS from membrane samples. The extract was subsequently diluted with methanol to a final concentration within the calibration range (0.1-25 ng/ mL) prior to LC/MS/MS analysis. XPS analysis of the isopropyl alcohol soaked membrane samples revealed that the amount of PFOS remaining on the membrane was insignificant.
3. Results and Discussions 3.1. Flux Performance. The flux performance of three NF membranes (NF270, NF90, and DK) and three RO membranes (ESPA3, BW30, and LFC1) under a constant applied pressure of 1379 kPa (200 psi) are shown in Figure 1a. Permeate fluxes decreased slowly over time, and relatively stable permeate fluxes were achieved in 4 days in all cases (
