Cross-sectional analysis of fouled SWRO membranes by STEM–EDS

Desalination 333 (2014) 118–125

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Cross-sectional analysis of fouled SWRO membranes by STEM–EDS Cyril Aubry a,⁎, Muhammad Tariq Khan a, Ali Reza Behzad b, Dalaver H. Anjum b, Leo Gutierrez c, Jean-Philippe Croue a a b c

Water Desalination and Reuse Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia Advanced Nanofabrication and Imaging Core Lab, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia Environmental Engineering, The Center of Advanced Materials for the Purification of Water with Systems, University of Illinois at Urbana-Champaign, USA

H I G H L I G H T S • • • •

Cross-sections of fouled RO membranes have been obtained with FIB. Cross-sections have been analyzed with the STEM/EDS line profile technique. Internal structure and elemental composition were described. STEM/EDS resolution was improved to detect 25 nm long structures.

a r t i c l e

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Article history: Received 6 April 2013 Received in revised form 13 November 2013 Accepted 17 November 2013 Available online 15 December 2013 Keywords: Reverse osmosis membrane Fouling layer STEM FIB EDS

a b s t r a c t The intact cross-section of two fouled reverse osmosis membranes was characterized using a scanning transmission electron microscope (STEM) equipped with an electron energy dispersive spectroscope (EDS). Focused ion beam (FIB) was used to prepare a thin lamella of each membrane. These lamellas were then attached to a TEM grid for further STEM/EDS analysis. The foulant in sample A was mainly inorganic in nature and predominantly composed of alumino-silicate particles. These particles were surrounded by carbon at high concentrations, indicating the presence of organic materials. Iron was diffusely present in the cake layer and this could have enhanced the fouling process. The cake layer of membrane B was mainly consisted of organic matter (C, O, and N representing 95% of the total elemental composition) and organized in thin parallel layers. Small concentrations of Si, F, Na, Mg, and Cl were detected inside the active layer and support layer of the membrane. Due to the high sensitivity of the cake layer of membrane A to the electron beam, STEM/EDS line analyses might have been performed on large areas. On the other hand, the cake layer of sample B was resistant to the electron beam and the resolution of STEM/EDS was gradually improved until obtaining a resolution of 25 nm. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Reverse osmosis (RO), nanofiltration (NF), and microfiltration (MF) are the commonly used membrane filtration processes in water reuse and purification systems to produce water for drinking, industrial, and agricultural purposes [1–7]. However, a major drawback in RO, NF, and MF systems is membrane fouling. Precipitated salts (scaling), organic materials (OMs), and microorganisms (biofouling) gradually accumulate on the surface of membranes leading to the formation of a cake layer [8–12]. The fouling mechanism is complex and involves different interactions between foulant and membrane surface. These interactions are influenced not only by the feed water characteristics (i.e., microbial composition, organic contents, pH, ionic strength, presence of multivalent cations, and temperature) and membrane surface structure but also by operating conditions (i.e., flux, percent recovery, and ⁎ Corresponding author at: KAUST, PO2850, 23955-6900 Thuwal, Saudi-Arabia. E-mail address: [email protected] (C. Aubry). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.11.033

cross flow velocity) and foulant characteristics [13–17]. The result is the development of a complex and structured foulant layer that negatively affects the membrane efficiency (i.e., loss of permeability, increase in hydraulic resistance, and decrease in rejection of contaminants) [18–20]. A detailed understanding of membrane fouling mechanisms, including an exhaustive foulant characterization (i.e. structural features and elemental composition), is required to develop membrane systems less susceptible to fouling. A powerful tool routinely used to investigate membrane failure and fouling mechanisms is morphological analysis [21,22]. Specifically, membrane morphological changes before and after fouling are examined by microscopy techniques for direct visualization of surface features and cross-sectional details. Topographical images by atomic force microscopy (AFM), and high resolution micrographs by scanning electron microscopy (SEM) and environmental SEM (ESEM) have allowed a better visualization and understanding of membrane pore structure, surface coverage, and pore constriction by foulants [23–27]. In addition, these analyses provide information about the type and

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exact adsorption location of foulant, and their spatial distribution on the membrane. However, many methods currently used to prepare membranes for surface and cross section analyses suffer from loss of material from the cake layer during the specimen preparation process because of the application of chemicals. In addition, due to the asymmetric nature of the membrane, artifacts and structural deformation of cake layer and membrane are unavoidable during mechanical cross-sectional cutting [28–33]. For example, Tang et al. (2006) embedded humic-fouled RO membranes in resin and used microtome to prepare cross-sections and characterized them by transmission electron microscopy (TEM) [34]. Focused ion beam (FIB) technique is commonly used to prepare cross section of samples for internal morphological analysis by TEM and elemental speciation along the cross section with energy dispersive spectroscopy (EDS). Friedmann et al. (2011) and Thompson et al. (2012) used FIB cross-sectioning technique to investigate interactions between cells and nanoporous aluminum oxide (alumina) membranes, and to measure the thickness of a cake layer deposited on a reverse osmosis polyamide membrane, respectively [35,36]. In this well described method, FIB excavates the sample by sputtering the matrix without using any chemical reagent or mechanical action [37–40]. The purpose of this study was to prepare thin lamellas from fouled membranes to examine the cake layer ultrastructure with TEM. The STEM/EDS line profile was used to analyze the elemental composition of the cake layer. Finally, the method described in this investigation generated minimal structural damage to the cake layer and membrane.

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of membrane) of membrane A (3.43 g/m2) was nearly 30% lower than that of membrane B (4.35 g/m2). Pretreatment setup at this plant included chlorination (0.5–0.6 ppm) at the intake, ultrafiltration (UF) unit, dechlorination with sodium bisulfite (1.0–1.5 ppm), and polymeric antiscalant dosing steps before RO. Water parameters for sites no1 and no2 are summarized in Table 1. Unfortunately, when fouled membrane coupons were harvested no virgin membrane samples were available. Therefore, the characterizations of the virgin membrane could not be performed. 2.2. Loss on Ignition (LOI) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) analyses Foulant material from fouled membrane was isolated by physical scrapping followed by a lyophilization process. Estimation of inorganic/ organic contents was conducted by a Loss on Ignition (LOI) test [41]. Additionally, elemental analysis of the acid-digested (HNO3) foulant material was performed by ICP-OES (Varian, USA) [42]. Moreover, further characterization of the foulant materials was also conducted with other techniques (i.e., Solid State 13-C Nuclear Magnetic Resonance (NMR), Pyrolysis Gas Chromatography–Mass Spectrometry (GC–MS), Fourier Transform Infra Red spectroscopy (FTIR), Adenosine TriPhosphate (ATP) test method, and Phyologenetic analyses). Protocol description and discussion of data related to these techniques can be accessed in our recently published study [43]. 2.3. Membrane freeze-drying

2. Materials and methods 2.1. Membranes and raw water sources This study was conducted on two thin film composite (TFC) polyamide seawater reverse osmosis (SWRO) membranes named as A and B (specifications of the membranes were not provided by the water treatment company). The membranes were received as 8" spiral wound modules harvested from a full scale plant (capacity of 53,000 m3/d) mounted on two floating barges. Membrane A was first operated for one year in the southern region of Saudi Arabia (herein site no1), where the barges were anchored in a very shallow channel (approximately 4 m in depth). Intake water quality was characterized by high turbidity (up to 15 NTU) due to inclusion of silt and sand from seabed. Barges were then moved towards North on the Red Sea coast (site no2) with the intake point located in open sea at approximately 20 m from the shore and several meters in depth (turbidity 0.3 NTU). Membrane A was operated for an additional 2 month period at this location (site no2). Membrane B was operated only at site no2 for 2 months before it was harvested. Both modules were operated as lead element. Fouling load (foulant mass per unit area

Table 1 Fundamental parameters of water quality. Quality parameter

pH Cond. (ms/cm) TDS (g/l) Turbidity (NTU) SDI15 TOC/DOC UV254 Abs. HPC (CFU/ml)

Site A

Site B

Raw SW

Raw SW (Std. Dev.)

SWRO Inlet (Std. Dev.)

– – 61.78 Up to 15 – – – –

8.23 61.6 40.65 (2.1) 0.5 ~5.0 1.20/1.12 0.007 6.0 × 103 (1.2 × 102)

8.21 61.5 – 0.1 1.2 (0.08) 1.08/1.06 0.007 1.18 × 102 (1.6 × 101)

SWRO: seawater reverse osmosis. SW: seawater. Std. Dev.: Standard Deviation.

To remove water present in the sample and to preserve the structure of the cake layer with minimum structural damage or distortion, membrane samples were freeze-dried using a K775X turbo freeze dryer (Quorum Technologies, UK). The freeze dryer was equipped with a high vacuum pump (1 × 10−5 mbar), a stage cooled by liquid nitrogen, and a heater to adjust the temperature of the stage. The freeze-drying procedure included the following steps: 1) freezing of samples in slush nitrogen; 2) transference of samples to the freeze-dryer stage set at −120 °C; 3) primary drying to remove the bulk water by sublimation (i.e., conducted by increasing the stage temperature from −120 °C to 0 °C over a period of 24 h, where approximately 90% of water is removed); and 4) secondary drying to remove the remaining water associated with macromolecules from unfrozen hydration shell water (i.e., conducted by further increasing the stage temperature from 0 °C to 25 °C for another 24 h). 2.4. Membrane sample preparation for TEM analysis A Quanta 3DFEG dual beam SEM (FEI Company, Netherlands) was used to prepare a thin cross-section slice (typically 100–200 nm thick lamella) of the freeze-dried membrane for STEM/EDS analyses. The dual beam SEM is equipped with a micromanipulator (5-axis motorized stage AutoProbe 200, Omniprobe, UK) for sample handling and a gas injection system (OmniGIS, Oxford Instruments, UK) for platinum deposition. Initially, the freeze-dried membranes were sputter-coated with at least a 65 nm thick gold layer using a K575X sputter coater (Quorum Technologies, UK) [44]. This coating protected the cake layer from ion beam-induced damaged during platinum deposition. Subsequently, the area of interest on the membrane surface was coated with a 25 × 2 × 2.5 μm (length × width × thickness respectively) platinum band. This protective band was necessary to avoid any sputtering of the cake layer during the ion beam milling and also to dissipate the heat brought by the ion beam. Platinum coating is obtained from a precursor organometallic gas ((CH3)3CH3C5H4Pt) introduced close to the surface while the ion beam activates the deposition by cracking the adsorbed molecules. After platinum deposition, the ion beam was used to cut 10 μm in depth trenches on either side of the platinum

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band. Further etching on either side of the platinum band was conducted to reduce the width of the lamella to 1 μm. In order to protect the fragile and brittle freeze-dried cake layer, additional platinum bands were deposited on the vertical side of the lamella. The lamella was partially cut free using the ion beam and then the micromanipulator needle was soldered to the top of the lamella using platinum deposition. The lamella was then completely cut free with the ion beam, lifted out from the membrane and attached in situ to a TEM grid. Following attachment, the grid was released from the micromanipulator needle by ion milling. Finally the lamella was thinned out to 100 nm in thickness with a low current ion beam. Fig. 1 shows the different steps of this procedure.

2.5. TEM and STEM–EDS analyses TEM and STEM–EDS characterizations were performed on a Titan G2 80–300 keV cryo-twin (CT) microscope (FEI Company, Netherlands). This transmission electron microscope can also operate in scanning TEM (STEM) mode and is equipped with an energy dispersive spectroscopy analyzer (EDAX, US). STEM signal was collected with a high-angle annular dark-field detector (HAADF) attached above the camera chamber. EDS was collected on 2040 channels (each channel of 10 eV) and the acquired data was quantified by Gatan Microscopy Suite software (GMS 1.8) using the Cliff–Lorimer approximations (hence errors in elemental composition values were ±10–20%). Each dot of the atom line profile represented the average signal collected during the scan of one area (scanning conditions will be discussed further). The atom line profile was then obtained by scanning successive squares until forming a line. Energy dispersive spectroscopy (EDS) uses an electron beam to generate X-ray fluorescence on the sample. The primary electron ejects an electron from the inner shell of an atom freeing an energy level. To fill this vacancy an electron from a higher level drops to this level and an X-ray, whose energy corresponds to the difference between the energy of the two levels, is emitted. As each element of the periodic table owns specific energy level distribution, each element has an explicit X-ray spectrum and so can be easily identified. Due to its energy, the primary electron is not stopped and continues its trajectory inside the matter and generates other X-ray emissions by ejecting inner electrons from neighboring atoms. In consequence the X-ray emission is not only limited at the surface but also includes a volume around the beam impact. The size of this volume depends mainly on the energy of the primary electron beam and the density and the thickness of the sample. Spatial resolution of STEM/EDS is directly correlated to this volume as the smallest details that could be identified are expected to be in this volume range size. To estimate this volume, simulations were performed using CASINO (monte CArlo SImulation of electroNs trajectory in sOlids, University of Sherbrooke, Québec, Canada) software [45]. Simulations

were conducted with a 300 keV and 5 nm radius electron beam on a 200 nm thickness layer of carbon and alumina-silicate, where the data was obtained with 50 thousand electrons. 3. Results and discussion 3.1. Chemical composition and nature of the fouling layer Prior to STEM/EDS analysis, inorganic content and elemental composition of the fouling layer of the two membranes were determined through the LOI test and ICP-OES analysis, respectively. The use of these two procedures for inorganic characterization of foulant material is a conventional but indirect approach. The fouling profile of membrane A, attributed mainly to the sediment inclusion in the feed water at site no1, was totally different compared to that of membrane B. The foulant material of membrane A consisted of 91.77% inorganics with Si and Al as the major elements. Conversely, foulant material of membrane B was composed of 75.75% organics and only 24.25% of inorganics with Ca and Na as the main inorganic elements (Table 2) [43]. 3.2. STEM–EDX resolution The electron trajectories and X-ray radial distribution obtained with CASINO software are presented in Fig. 2 left and right respectively. Both electron diffusion and X-ray generation lengths were estimated to be 20 nm around the impact point of the electron beam on the surface sample. Therefore, the resolution of the line profile can be estimated as equal to the scan size plus twice the diffusion length (i.e. 40 nm). Because the EDS line profile analysis was performed on a small surface area (few square micrometers) of the sample, it only represents the elemental composition of that particular spot. Consequently, the EDS line results represented a local variation in the global composition of the foulant material. On the other hand, ICP-OES analysis was conducted on 10–20 mg of homogenized foulant material after acid digestion, therefore resulting in the overall/average concentration of the elements in the foulant material. 3.3. STEM view of sample A Fig. 3 shows a STEM micrograph of sample A in dark field mode. This image clearly shows the platinum and gold layers used to protect the sample, along with the fouling cake layer and the polyamide active and polysulfone support layers of the membrane. The fouling layer is displayed intact indicating that the use of FIB and micromanipulator to pick up a thin slice was appropriate to preserve the structural integrity of the sample. The fouling layer appears rough and has a thickness

Fig. 1. TEM sample preparation. First: titled view of the sample with gold and platinum layers. Second: sample has been etched, cut and ready to be lift-out with the micromanipulator needle. Third: the sample is attached to a TEM grid.

C. Aubry et al. / Desalination 333 (2014) 118–125 Table 2 Estimation of organics/inorganics fractions and ICP-OES elemental analysis of the membrane foulant material.

Membrane A

Membrane B

Loss On

% LOI

8

76

Ignition

% Residue

92

24

ICP-OES Analysis

Al

119

4

Ca

10

182

Fe

64

2

Mg

21

25

84

103

Na

mg/g

P

2

9

S

33

25

Si

503

96

Zn

1

47

in the range of 1.2–2 μm, resembling a layer of deposited particles or debris rather than a compact and homogeneous matrix. In addition, voids are clearly observed in the fouling layer. Different studies have already reported this kind of structure of fouling layer when RO membranes are used to treat seawater. The presence of debris can be attributed to particles (e.g., sand, clay, etc.) which would not have been stopped by the pre-treatment step [7]. Hence, these particles would interact with each other or with the matter close to the surface of the membrane to form a cake layer [46,47].

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3.4. STEM–EDS line profile of sample A Spectrum imaging datasets were acquired in 50 nm in scan size and by dwelling the beam for 33 s at each pixel to obtain optimal EDS signal. The resolution of the atom profile was 90 nm (i.e. 50 nm scan size + 2 × 20 nm diffusion length). Fig. 4 shows two EDS line profiles performed on two different locations of the cake layer with their associated atom profile. Dark lines presented on the surface correspond to the impact of the electron beam during EDS line profile (scanning direction is from the left to the right), pointing out that the energy of the electron beam was sufficient to alter the cake layer (i.e., knock-on damages or atom displacements caused by the inelastic collisions between incident electron beam and atoms of the sample [48]). This result suggests that the resolution of the STEM–EDS cannot be improved (i.e. reduction of the size of the scanned area) because it will bring more energy and consequently generates more damage to the sample. In the two profiles the cake layer and the membrane are clearly identified by the sudden change in the elemental composition. No diffusion of species into the membrane could be measured. Two line profiles showed that Si, Al, and O were dominant elements (more than 80% of the total composition) along the entire thickness of the cake layer. The inorganic nature of the foulant is confirmed by LOI and ICP-OES analyses and can be attributed to the presence of aluminum-silicate particles. Iron was also present along the cake layer with a relative abundance fluctuating from 0% to 15% (i.e., average value 8.75%). Carbon was not abundant except for the presence of two peaks, one at the surface of the cake layer associated with titanium, and the other close to a particle, where the percentage of carbon reached 27% and 20%, respectively. In the second half of the cake layer, carbon was present and its relative abundance didn't exceed 5%. Traces

Fig. 2. CASINO simulations of electron trajectories and X-ray emission spatial distribution during the interaction between a 300 kV electron beam and a 300 nm thick layer of carbon (top) and Al2O3 (bottom).

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Fig. 3. STEM micrograph of the cross-section obtained from membrane A. Different layers as well as STEM–EDS profile are mentioned.

of Ti, K, P, and S were detected. Conversely, Ca, Na, and Cl have not been detected. Fig. 4a shows a STEM view of a region of the cake layer with the correlated atom profile. A particle of large size (labeled as particle 1) was clearly observed and it was mainly composed of alumino-silicates (higher signals for Al, Si, and O). A relatively smaller particle (labeled as particle 2) cut by the ion beam, was present on the left side of particle 1. The elemental composition of particle 2 was very similar to that of particle 1. Relatively higher signals of carbon close to the surface of both particles were observed, which might be an evidence of organics deposited on the surface of alumino-silicate particles [49–51]. Relatively strong signals for Fe were observed on the boundaries of these two particles. This ion (i.e., Fe) might have played an important role in bringing together negatively-charged alumino-silicate particles. On the right side of particle 1, a thin flat bright structure was observed very close to the membrane surface. The composition of a whitish structure was slightly different compared to that of particles 1 and 2 since it also showed the presence of Fe and K (i.e., ca. 5% and ca. 2%, respectively), whereas the relative abundance of Si was lower (i.e., ca. 25%). The relatively different shape and chemical composition of this particle suggested that either it was another kind of aluminum-silicate or it might be an aggregate of smaller alumino-silicate particles adhered

together when their electrostatic repulsion was decreased by positively charged ions (i.e., Fe and K). The concentration of Fe also increased near the negatively charged membrane surface at seawater pH [52]. This direct analysis of elemental profile changing along the cross section of the cake layer showed that the membrane was mainly fouled by alumino-silicates with the contribution of cations (i.e., especially Fe). Cations are known to mitigate the electrostatic repulsion between negatively charged species, which are then attached together due to attractive Van der Waal's forces. The role of Fe in the silica fouling of SWRO membrane has been reported by Sahachayunta et al. (2002) and Cob et al. (2012 and 2013) [53–55]. Nitrogen (N), commonly found in biogenic matter (i.e., especially proteins) could not be detected. Also, no microbial structure could be identified. This result suggested that membrane A was not significantly impacted by biofouling. The EDS profile (Fig. 4b) indicated the presence of iron-enriched aluminum silicate particles mixed with traces of carbon. The presence of other elements was less pronounced (i.e., below 1% in relative abundance) and, due to sensitivity limitations of the technique and presence of noise, it was difficult to precisely determine their abundance. The only observation was that K and S were present from the top surface of the fouling layer to the membrane surface.

Fig. 4. a and b. TEM micrograph after STEM–EDS line profiles (indicated with blue arrow) of sample A and their corresponding atom profile.

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Fig. 5. Left, dark field TEM micrograph of sample B cross-section with 90 nm resolution STEM–EDS line profile indicated with a blue arrow. Right, zoom on the region indicated with the green square and the 50 nm resolution EDS–STEM line profile is showed with a blue arrow.

STEM–EDS line profile was performed on the intact part of the cake layer with the same parameters as for sample A (i.e., resolution of 90 nm) and data is shown in Fig. 6. Different regions of the sample,

cake layer (CL), polyamide (PA), and polysulfone (PS), are symbolized with rectangles. The fouling layer was mainly organic in nature (C, O, and N represent approximately 95% of the total elemental composition) as indicated by the LOI analyses, i.e., 75% of the matter was found to be organic in nature (Table 1). Along the cake layer towards the membrane surface, the EDS line profile showed that the relative abundance of carbon gradually increased (i.e., from 70% to 80%) while the relative abundance of oxygen and nitrogen decreased (i.e., from 14% to 8%, and from 6% to 2%, respectively). The higher abundance of C and lower abundance of N near the membrane surface suggested that the initial fouling layer (conditioning film) was constituted mainly by non-nitrogenous compounds, e.g., polysaccharides [56]. Nitrogen abundance was gradually increased from initial (i.e., near the membrane surface) to top fouling layer. This is an indicative of increase in proteinaceous structures, (e.g., microbial cell contents, extra cellular polymeric substances (EPS) rich in proteins) with the age of the fouling layer. Others elements (i.e., F, Cl, S, Na, Mg, Al, Si, and P) showed a smaller (i.e., b4.5%) and relatively constant abundance. With the exception of C, N (increased in PA region), and S (increased in PS region), the abundance of all other elements decreased inside the membrane. C, O, N, and S are known as constituents of membrane

Fig. 6. Atom line profile of sample B obtained with 90 nm resolution STEM–EDS line profile. Each region of the sample was denoted as CL: cake layer, PA: polyamide and PS: polysulfone.

Fig. 7. Atom line profile of sample B obtained with 50 nm resolution STEM–EDS line profile. Each region of the sample was denoted as CL: cake layer, PA: polyamide and PS: polysulfone.

3.5. STEM on sample B The STEM micrograph of sample B in dark field mode is presented in Fig. 5. The large voids present indicated that the sample has been ripped during preparation, therefore they were not considered as representative of the structure of the fouling layer. Nevertheless, the thickness of the fouling layer could be estimated as 1 μm where the lower part of the cake layer remained intact and the STEM–EDS analysis was possible to be performed. The cake layer structure of membrane B appeared to be very different compared to that of membrane A. This layer showed a compact structure with neither voids nor particles present. The cake layer was composed of a number of successive thin layers with thicknesses in the range of few nm.

3.6. STEM–EDS line profile on sample B

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material. The presence of elements other than C, O, N, and S in the membrane structure could be attributed to either diffusion into the membrane, or contamination occurred during sample handling. These elements could also be incorporated into the membrane structure during the manufacturing process [57]. To settle, STEM/EDS analyses should have been done on new membranes just before water treatment process. STEM micrograph (not shown) of the sample after STEM–EDS measurements showed that, unlike sample A, the exposure of the cake layer to the electron beam did not significantly alter its structure. This indicates that this cake layer is more resistant to the electron beam and confirmed that organic compounds are less impacted by knock-on damages than inorganic compounds when using a 300 keV electron beam [48,58]. In addition, the STEM–EDS measurement at higher resolution was possible without any major structural damage to the cake layer. Considering that the STEM–EDS spectrum is obtained by scanning the specimen surface with an electron beam of 5 nm in diameter, scan area was not reduced to less than 10 nm. As a consequence, the scan resolution was approximately 50 nm. These parameters were used to perform STEM–EDS on sample B (dwelling time 36 s for optimal signal) and EDS line profile is presented Fig. 7. Except for the presence of calcium and potassium, the global composition of the cake layer was similar to that obtained with a lower resolution. A sub-structure was clearly identified near the membrane surface by the EDS profile showing sudden increase in the signal for P (from 2% to 6%), O (from 5% to 9.5%), Mg (from 1.8% to 3.2%), Na (from 1% to 2%), and F (from 1.7% to 2.5%). The size of this sub-structure was approximately 25 nm, corresponding to half of the estimated resolution of this technique (50 nm). This structure could be a phosphate particle with adhered cations. The detailed characterizations of these two fouled membranes by different chemical and microbiological techniques, have been discussed in our previous work [43]. Results indicated that the foulant material of membrane A was inorganic in nature and largely composed of aluminum and silicon compounds. Conversely, the foulant material of membrane B was predominantly organic in nature, comprising microbial cells and microbial contents/exudates (i.e., polysaccharides, proteins, lipids, and nucleic acids). The STEM/EDS line profiling of different elements across the thickness of cake layer not only confirmed these findings but also suggested possible mechanisms of fouling layer formation. 4. Conclusion Freeze drying of fouled membranes coupled with FIB milling is a suitable sample preparation technique for TEM and EDS characterizations because it preserves the nature of the cake layer. Analysis of foulant layer by ICP-OES and LOI techniques confirmed that the nature (i.e., organic or inorganic) of the fouling layer is not altered during sample preparation. Nevertheless, the freeze-dried cake layer remained fragile and brittle, and still could be easily damaged during handling and transportation. STEM/EDS provided relevant information about fouling mechanism and cake layer internal structure and composition. STEM–EDS is considered as a direct membrane fouling characterization technique (i.e., analysis of intact fouling layer), which generates information consistent with the results obtained by other indirect analytical techniques (i.e., analysis of isolated foulant material). The advantage of this technique is that it enables the observation of the internal morphology of the cake layer and the spatial distribution of different foulant species, which consequently leads to the elucidation of specific foulant–foulant interactions taking place in the overall fouling phenomenon. References [1] C. Bellona, et al., Comparing nanofiltration and reverse osmosis for drinking water augmentation, J. Am. Water Works Assoc. 100 (9) (2008) 102-+.

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