Desalination 305 (2012) 44–53 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Structural and chemical characterization of long-term reverse osmosis membrane fouling in a full scale desalination plant N. Melián-Martel a, b,⁎, 1, J.J. Sadhwani b, S. Malamis a, c, M. Ochsenkühn-Petropoulou a,⁎⁎ a b c Laboratory of Inorganic and Analytical Chemistry, School of Chemical Engineering, National Technical University of Athens, Iroon Polytechniou 9, 15773 Athens, Greece Department of Process Engineering University of Las Palmas de Gran Canaria, Campus de Tafira Baja, 35017, Las Palmas de Gran Canaria, Spain Department of Biotechnology, University of Verona, Strada Le Grazie 15, I-37134, Verona, Italy H I G H L I G H T S ► ► ► ► ► Research area includes structural and chemical characterization of membrane fouling and analytical methods. Combined analyses provide complimentary data for membrane fouling interpretation. Appropriate use of the techniques can improve the performance and reduce the operational costs of membrane plants. AAS, ICP-OES and IC provide valuable quantitative results concerning the spectrum of foulant elements. The main inorganic foulants on RO membranes consist of Si, Al, Cl, Ca, Na, Mg and K and minor trace elements. a r t i c l e i n f o Article history: Received 2 May 2012 Received in revised form 3 August 2012 Accepted 6 August 2012 Available online 1 September 2012 Keywords: Reverse osmosis Membrane fouling Chemical and structural characterization Membrane autopsy Desalination plant a b s t r a c t The assessment of long-term fouling in reverse osmosis (RO) membranes was investigated through extensive membrane autopsy using different analytical techniques. The RO membranes were taken from a seawater desalination plant after 4 years of operation. Chemical and structural characterization was performed using different analytical methods, including visual observation, optical microscopy (OM), scanning electron microscopy with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), inductively coupled plasma optical emission spectrometry (ICP-OES), atomic absorption spectroscopy (AAS) and ion chromatography (IC). The fouling layer consisted of particulate matter embedded in an apparently amorphous matrix, which was unevenly distributed over the membrane surface with the greatest accumulation in the valley areas. Inorganic and organic foulants were identified in the RO membranes. Inorganic foulants mainly consisted of hydrogen aluminosilicates and halite. The main organics substances identified were proteins, polysaccharides and humic compounds and were attributed to biofouling. Chemical analysis revealed that Si, Al, Cl, Ca, Na, Mg and K were the predominant elements contributing to membrane fouling. The reasons for the deposition of foulants on the RO membranes are the increase of membrane selectivity due to biofouling, the large size of cartridge filters and the high operating pressure. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Seawater reverse osmosis (SWRO) membrane technology is increasingly being employed as a strategy for the conservation of limited fresh water resources. Despite its growing popularity and improved technology ⁎ Correspondence to: N. Melian-Martel, Department of Process Engineering University of Las Palmas de Gran Canaria, Campus de Tafira Baja, 35017, Las Palmas de Gran Canaria, Spain. Tel.: +34 660163130; fax: +34 928 458975. ⁎⁎ Corresponding author. Tel.: +30 210 7723094; fax: +30 210 7724039. E-mail addresses: nmelian@proyinves.ulpgc.es (N. Melián-Martel), oxenki@central.ntua.gr (M. Ochsenkühn-Petropoulou). 1 Temporary address: Laboratory of Inorganic and Analytical Chemistry, School of Chemical Engineering, National Technical University of Athens, Iroon Polytechniou 9, 15773 Athens, Greece. 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.08.011 [1], reverse osmosis (RO) along with all membrane separation processes are still hampered by the inevitable membrane fouling [2–5]. Potential causes of membrane fouling are well known and well documented and include: inorganic fouling (scaling), organic fouling, colloidal fouling and biological fouling or biofouling [6–10]. The accumulation or adsorption of foulants on the surface or into the membrane matrix results in the reduction of membrane performance over time, which requires costly pretreatment, higher operating pressures, and frequent chemical cleaning – which damage membranes, degrade permeate quality, and hasten membrane replacement – increasing water treatment cost and energy consumption [11]. Effective control of fouling requires a good diagnosis of the type and extent of fouling present in order to maintain plant performance [6,12] since each foulant requires different means of removal. Pontié et al. [6] and Vrouwenvelder et al. [12] have employed analytical tools for N. Melián-Martel et al. / Desalination 305 (2012) 44–53 diagnosis, prediction, prevention and control of fouling on membranes through the autopsy of the nanofiltration and RO membrane elements [6,12]. Several autopsies of RO fouled membranes have also been carried out in order to identify the species of foulants and to better understand the physicochemical processes governing fouling [13–18]. Currently, there is very little data available concerning the characterization of long-term fouling in full scale RO plants. Furthermore, there is no standard analytical scheme method available for studying fouled membranes that can be used for fouling diagnosis without the need of special experience in order to interpret the obtained results [19]. The difficulty in characterizing fouling is often attributed to the complexity of feed water composition and to the different fouling mechanisms of the different foulants [13]. In this work, a methodology was developed based on analytical tools and methods for the characterization of long-term fouling in RO membranes taken from a full scale desalination plant operating for 4 years. This case study can reveal significant issues concerning long-term RO membrane fouling. The approach is based upon the interpretation of the structural and the chemical characterization of membrane fouling in actual seawater operating conditions by means of autopsy of a virgin (non fouled) and fouled spiral wound RO membrane elements and comparison of the findings with the literature. The analysis of the virgin membrane properties and membrane foulant characterization enabled the determination of the foulants that are responsible for shortening the useful life of the membranes in a commercial RO desalination plant. A systematic investigation based on the application of an autopsy of virgin and fouled membrane elements has been carried out, which enables integral diagnosis of the type and extent of fouling. 2. Experimental 2.1. Investigated plant The investigated fouled membranes were taken from the Arucas– Moya desalination plant located in the Northern coast of the island of Gran Canaria (Canary Islands). The SWRO desalination plant was constructed in 1995 in order to supply potable water to the region. The SWRO has a production capacity of 15,000 m3/day, where the seawater intake is conducted by means of six coastal wells drilled in solid rocks. In this area pH is rather neutral (7.55), but some sulfuric acid is required for pH adjustment. The seawater temperature is around 20 °C with silt density index (SDI) values below 1 and less than 1 NTU of turbidity. The chemical analysis of the feed seawater is presented in Table 1. In the pre-treatment section, the seawater is chlorinated at the head of the intake pipe using sodium hypochlorite solution preventing biological growth. Seawater is filtered by four sand pressure filters. After Table 1 Chemical analysis of the feed seawater of the region [20]. Main ions and compounds mg/l Potassium (K+) Sodium (Na+) Magnesium (Mg2+) Calcium (Ca2+) Carbonate (CO32−) Bicarbonate (HCO3−) Chloride (Cl−) Fluoride (F−) Sulfate (SO42−) Silica (SiO2) Boron (B3+) 483.49 10,995.20 1589.77 443.70 36.39 97.63 20,566.80 1.47 2900.00 0.14 5.00 Other parameters TDS (mg/l) T (°C) 37,143 25 45 filtration the water passes through a series of four cartridge filters, which retain particles greater than 5 μm. Sodium metabisulfite is added in order to neutralize any remaining chlorine in the feed water and then the seawater is fed to the RO system. The RO unit section consisted of two racks (2 × 7500 m 3/day, 45% recovery), which work in a 1 step–1 stage system, having a total of seven elements per pressure vessel. The schematic flow diagram of the desalination plant is shown in Fig. 1. The membranes used in the process were spiral wound RO membranes made of polyamide (FILMTEC™ SW30HR-380) operating at 68 bar. Usually about 5–8% of the membranes are annually replaced in order to maintain the targeted product quality as well as the permeate quantity. The membranes are chemically cleaned every 3 months. The replacements of RO elements are required since otherwise the permeate quality would be lower and the operational pressure would increase. 2.1.1. Membrane sampling In this study one virgin and two fouled spiral wound RO membrane elements were autopsied. The fouled membrane elements were removed from the plant (first element in the pressure vessel), and were opened as soon as possible to minimize the possibility of biological growth in the module. The fouled membrane elements were kindly provided, in a dry condition, as 30 × 30 cm flat sheets. Among each fouled membrane elements tested, four sample sheets were taken using random samples. At the virgin membrane element a dissection was conducted using a circular power saw to remove the ends and slice through the fiberglass sleeve. The leaves were unrolled, and samples were collected from a region approximately at the middle of the module. The fouled RO membrane element selected for the autopsy study had been in service for nearly 4 years once they were removed for inspection and analysis. Table 2 provides an overview of the operational specifications of the RO membrane elements that were recommended by the manufacturer. It is made of polyamide thin-film composite (thickness b 1 μm) that is supported by polysulfone layer (thickness ~ 140 μm) configured by spiral-wound type with high salt rejection efficiency [22]. For comparison, the same structural analysis and chemical characterization protocol were applied to both virgin and fouled membrane samples. 2.2. Approaches for structural analysis and chemical characterization The analytical techniques used in the structural analysis and chemical characterization of the membrane include: visual observation, optical microscopy (OM), scanning electron microscopy with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), inductively coupled plasma optical emission spectrometry (ICP-OES), atomic absorption spectroscopy (AAS) and ion chromatography (IC). 2.2.1. Optical observation The surface morphology was observed by optical microscopy using a Zeiss Axiotech 100 HD light microscope for optical microscopy observation (50 ×, 100× and 200 ×). Various fractions of 4 cm 2 were taken from different leaves of the membrane elements for optical analysis. 2.2.2. SEM-EDX analysis Membrane surface structure, morphology and the elemental chemical analysis of the membrane samples were conducted using a FEI Quanta 200 scanning electron microscope (resolution 6 nm) equipped with an EDX-Genesis 4000 energy dispersive X-ray fluorescence spectrometer (SEM-EDX). The instrument was operated in low 46 N. Melián-Martel et al. / Desalination 305 (2012) 44–53 Fig. 1. Schematic flow diagram of Arucas–Moya SWRO desalination plant. vacuum mode at chamber pressure of 80 Pa. The accelerating voltage varied from 20 to 25 kV, depending on the sample. No pretreatment or coating was applied to the samples, which were secured to the specimen holder with adhesive conductive double-sided carbon tape. The top surface and the cross-section of small pieces, approximately 1 cm 2, were both analyzed. For the cross-section analysis, the small pieces were oriented perpendicular to the incoming light/ electron beam. 2.2.3. XRD analysis The structural analysis of the membranes was carried out by XRD analysis. XRD analysis was performed with a Siemens D5000 diffractometer using CuKα1 radiation with a graphite monochromator, in the 2-theta range from 2-80° (counting time of 1.0 s, wavelength 1.506 Å). The power conditions were set at 20 kV/30 mA. The evaluation of the XRD spectra was performed automatically using the DIFFRAC AT Search Program provided as an integral part of the diffractometer. The analysis was carried out in three different ways: (1) analysis of the whole membrane without any pre-treatment (2) analysis of the active layer after separation of the polysulfone sublayer of the membrane and (3) analysis of the inorganic fraction after decomposition by heating the membrane substrate at 485 °C as it was found by TG (Thermo Gravimetric analysis) in a Mettler Toledo TGA/SDTA 851e. The skin and the sublayer parts of the membranes studied have been separated by manual pealing using an adhesive double tape. Table 2 Characteristics of RO membranes investigated [22]. Parameter Value Membrane Material Effective membrane area (m2) Permeate flux (m3/d) Salt rejection (%) Max. operating pressure (bar) Max. operating temperature (°C) pH range SW30HR-380 Polyamide thin-film composite 35 23 99.7 83 45 2–11 2.2.4. FTIR analysis Functional group characteristics were measured using a Jasco FTIR‐ 4200 spectrometer combined with an attenuated total reflectance device (ATR PRO 410-S with Ge crystal) employed for Fourier transform infrared (FT-IR) analysis of the virgin and fouled membranes samples. The scanning was carried out in the range from 500 to 5000 cm −1. Membrane swatches cut from the virgin and fouled RO membrane samples were placed in plastic petri dishes and were dried in a desiccator for 2 days. Both the membrane and the fouling layer were simultaneously analyzed. No pre-treatment was required for the samples. 2.2.5. XRF analysis The objective of this semi-quantitative analysis was to determinate the elemental composition of the virgin membrane and the fouling layer on the membrane and use the results to establish, as far as possible, the chemical nature of the deposits. An ARL ADVANT XP sequential XRF spectrometer was used to analyze the samples. The specimens were placed in glass sample bottles to preserve the fouling layer, were dried in a desiccator for 2 days without any preparation. The sample base was prepared by compressing boric acid in a hydraulic press at a pressure of 100 MPa for 15 s. The samples were then deposited on the boric acid disk prior to analysis by X-ray fluorescence. The results were analyzed using the semi-quantitative Quantas Software. 2.2.6. ICP-OES and AAS analysis The objective of using the ICP-OES and AAS techniques was to obtain, as far as possible, a quantitative analysis of the elements constituting the deposits, unlike the semi-quantitative XRF technique which mainly analyzes the surface of the sample. In order to extract the deposits, a piece of approximately 3 cm2 of each sample was accurately weighed and fragmented into smaller fractions, which were digested in 1:1 suprapure 30 wt.% HCl (Merck, Darmstadt) on a hotplate at approximately 75 °C for 15 min, thus enabling the complete leaching of the deposits from the membrane, shown by the color changing of the virgin membrane. Next filtration of the solution was performed through 0.45 μm cellulose nitrate 47 N. Melián-Martel et al. / Desalination 305 (2012) 44–53 membrane filters (Whatman). The final solution was diluted up to a final volume of 50 ml. A stock ICP multi-element standard solution IV (Merck, Germany), having a concentration of 1000 mg/l of each of the following elements: Ag, Al, B, Ba, B, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, K, Mg, Mn, Na, Ni, Pb, Sr, Ti, and Zn, was used to prepare the working standard solution (0.1–5 μg/ml). These standards were prepared in 0.1 M HNO3 using suprapure 65 wt.% HNO3 (Merck, Darmstadt) and highpurity deionized water (HPW) (Easy pure II, LF ultrapure water system, Barnstead). Perkin Elmer Optima 7000 DV spectrometer was employed to determine the concentrations of the trace elements with the following conditions: RF Power 1450 W, nebulizer gas flow rate 0.7 l/min, plasma gas flow 15 l/min, nebulizer pump 1.50 ml/min, axial and radial plasma view. The results were analyzed using a Win lab 32™ software. Varian fast sequential atomic absorption spectrometer, model AA240FS, was used to measure the concentration of the major elements (Na, K, Ca and Mg) using the software Spectraa, version 5.1 Pro. 2.2.7. IC analysis The ion chromatography system consisted of a Dionex-BioLC GS 50 gradient pump and a sample injector with a 25 μl loop, which were coupled to a Dionex-BioLC ED50 conductivity detector. The chromatography parameters for the determination of the investigated anions and cations are presented in Table 3. Both the fouled and virgin membrane samples were weighed in order to obtain the mass concentration of particulate matter deposited on the membrane. The membrane samples were conditioned prior to weighting in a desiccator for 24 h. The sample was then leached in an ultrasonic bath (Ultrasonik, NEY) 28 H with 15 ml of HPW for 30 min. The procedure was repeated with additional 10 ml of HPW using the membrane sample residue. The leachate was filtered (0.45 μm, cellulose nitrate membrane filters, Whatman filters) and diluted up to a final volume of 25 ml. All reagents used were of analytical grade. Working standard solutions in the concentration range of 0.1–20 mg/l were prepared from multi-element anion chromatography standards (Ca2+, Mg2+, K +, Na+, Cl −, NO3−, SO42− and PO43−) of 1000 mg/l (No 700008, Fluka, Switzerland) by dilution in ultra pure water. Eluents for ion chromatography were prepared by dissolution of appropriate amounts of Na2CO3 (≥99%, Merck) and NaHCO3 (≥99.5%, Sigma−Aldrich) for anions and methanesulfonic acid (≥99.5%, Sigma-Aldrich) for cations, in HPW. 3. Results and discussion 3.1. Structural analysis The leaf of the membrane samples had an orange-brown hue. The deposits of the fouled membranes surface samples were encrusted with scale precipitates which were not easily removed. When the feed spacer was removed, the optical images of the membrane surface revealed that the surface of the fouled membranes appeared to be covered by a fouling layer having an orange-brown color that Fig. 2. Optical images showing a fouled membrane surface in regions near the spacer strands. Magnification ×50. was unevenly distributed over the surface with the greatest accumulation in the regions located: (1) in the valleys of rough membranes, leading to “valley clogging”, and (2) underneath or in the vicinity of the spacer strands. This fouling pattern was evident in the present study both with the optical and scanning electron microscopes. Figs. 2 and 3 show the optical and SEM image of the fouled membrane surfaces near to the strands, respectively. Apparently the roughness of the fouled membranes was formed by imprints of the permeate spacer, the main purpose of which is to promote eddy mixing [23]. The most likely cause for the fouling close to the spacers is related to high applied pressure that is occasionally practiced. During the membrane operation, fouling occurs and this results in an increase in the pressure drop between the feed and the reject stream. The increased pressure drop resulting for constant permeate quantity may accelerate fouling particularly in the vicinity of spacers. Roughness, with valleys and deeper spots in the valleys, facilitates accumulation of fouling, which ultimately decreases the permeate flux and also prevents the effective cleaning by “protecting” deposits from being removed [24,25]. When the surface of the membrane was magnified, the layer of deposits that covered the whole surface could be more clearly observed. Preliminary tests revealed that samples removed from different locations within one membrane or from different membranes within one element had a homogeneous appearance in SEM, such that the morphology of the surface of fouling layers could be well characterized by analysis of a single membrane sample per element. Comparison among the surface of virgin and fouled membrane samples is shown in Fig. 4. SEM images clearly show the remarkable differences between the surface morphologies of the two membranes samples. While the foulant layer on the fouled membrane surfaces consisted of particulate matter embedded in an apparently amorphous matrix in nature (Fig. 4a1), the virgin membrane appeared clean and quite smooth in surface (Fig. 4b1). Table 3 Ion chromatographic parameters for the analysis of the water soluble anions Cl−, NO3−, SO42−, PO43− and the cations, K+, Na+, Ca2+, Mg2+. Ions analyzed Stationary phase/suppressor Elution Conditions Detection limits (μg·l−1) Cl− NO3− SO42− PO43− K +, Na+ Ca2+ Mg2+ Ion Pac AS 14 (250 × 4 nm, 9 μm particle size) Suppressor: ASRS-ULTRA (4 mm) 3.5 mM Na2CO3 + 1.0 mM NaHCO3 Flow rate: 0.8 ml·min−1 Pressure: 984 psi Background conductivity: 17.20 μs IonPac CS 12A (250 × 4 mm, 8 μm particle size) Suppressor: CSRS-ULTRA (4 μm) 20 mM methanesulfonic acid Flow rate: 1.0 ml·min−1 Pressure:1090 psi Background conductivity: 1.82 μS Cl−: 20 NO3−: 60 SO42−: 40 PO43−: 120 K+: 90 Na+: 30 Ca2+: 80 Mg2+: 20 48 N. Melián-Martel et al. / Desalination 305 (2012) 44–53 Fig. 3. SEM micrograph of the fouled membrane surface near to the strands. Particles of many different sizes appeared, although, even with magnification level no crystalline structures were evident in any case, except sodium chloride (NaCl). These images also show that the foulant pattern on the RO surfaces is not formed in a compact layer and that an uneven fouling throughout the membrane surface was observed. The major contribution of SEM analysis was that it identified the deposits as being predominantly amorphous in nature. The chemical composition by EDX was confirmed later. Using the data base from the XRD program an initial search was made for every possible compound. The XRD diagram of the virgin membranes sample revealed the amorphous pattern in nature of the polyamides membranes showing broad peaks in the region 2θ = 15–30° [26]. The XRD diagram patterns of the fouled membranes were similar. Even though three different methods were employed by XRD it was only confirmed in all the samples that halite (NaCl) and hydrogen aluminosilicate, H(AlSi2O5) are the unique components of the crystalline phase deposits observed on the membrane. This was also confirmed by the dominance of Si and Al observed in the chemical analysis. The bulky deposits were amorphous and not decipher by the XRD technique that only indentifies compounds in their crystalline state. The inhibitory action of scale inhibitors distorts the crystal structure of the scale-formers [15]. Consequently, the XRD indicates that significant inorganic fouling due to the presence of hydrogen aluminosilicate and halite occurred. The majority of foulants could not be identified. These substances are present in the feed solution; to minimize inorganic fouling frequent chemical cleaning with suitable chemicals is required. To identify the organic matter present on the membrane surface, the analysis of the fouling layer and the virgin membrane surface was carried out by ATR-FTIR (Fig. 5). The spectrum of the virgin polyamide membrane is dominated by vibrational bands of the polysulfone support membrane with major bands in the region of 1500 to 1600 cm −1 assigned to polysulfonyl Fig. 4. SEM micrograph showing (a) the fouling layer on the fouled membranes and (b) a virgin membrane surface respectively and associated EDX analysis. 49 N. Melián-Martel et al. / Desalination 305 (2012) 44–53 Fig. 5. ATR-FTIR spectra of the virgin and fouled polyamide RO membranes. group in the porous polysulfone layer [21]. The major vibrational bands associated with the thin polyamide layer are the amide I (C_O) near 1660 cm −1 and the amide II (N\H) near 1540 cm −1 [27]. Other bands at 1607, 1488, and 1448 cm −1 are associated with the C_C ring vibrations of polyamide [27]. The peaks around 3300–3400 cm −1 can be assigned to free and hydrogen bonded N\H stretching [28,29]. The coated layer of this kind of membranes is aliphatic with significant amount of OH groups [30]. Therefore, the broad band around 3300 cm −1 is probably attributed to a stretching vibration of N\H and carboxyl (\COOH) groups of the polyamide layer, and potential alcoholic (\COH) groups in the coating layers [30]. New peaks in the spectra from the fouled membranes appear at the wavenumbers (1010–1040, 916, 1650 cm −1) where the most interesting changes in the spectra of the fouled membranes took place in 1010–1040 cm −1 and 916 cm −1 characteristics of the functional group C\O which indicates that polysaccharides or polysaccharidelike membrane foulants, were presented in the fouling layer [31,32]. Examination of other regions of the spectrum can clarify whether the foulants are polysaccharides. Polysaccharides contain a significant number of hydroxyl groups, which exhibit a broad rounded absorption band above 3000 cm −1. Polysaccharides are one of the main substances of extracellular polymeric substances (EPS) [33]. This adhesive polysaccharide material can act as trap for other organic debris as a source for further microbiological growth [34]. The band in the vicinity of 1400 cm −1 could be due to aliphatic C\H deformation, C\O stretching and O\H deformation of phenol [13]. The band in the range of 600–800 cm −1 could be due to aromatic compounds. These results suggest that the constituents of the membrane organic fouling included proteins, polysaccharides and aromatic compounds derived from humic compounds. The entrapment of organic material on the membrane and in its pores can result in the development of microorganisms and thus biofouling. Since chlorine is used in the feed water to eliminate microbial activity, microorganism development takes place after the removal of chlorine through the addition of sodium metabisulfite. The presence of proteins and polysaccharides most likely means that biofilm actually develops. Biofilm is detrimental to membrane performance as it increases the transmembrane osmotic pressure and increases the hydraulic resistance; this is referred to as ‘biofilm-enhanced osmotic pressure’. The contribution of EPS to fouling has been found to be significant [35]. To minimize biofouling, it is important to conduct frequent cleaning of the membrane with suitable chemicals as well as frequent flushing with permeate water. 3.2. Chemical characterization The results from the semi-quantitative XRF analysis are shown in Table 4. The sum of the elemental composition does not add to 100% since the concentration of carbon, nitrogen, and oxygen could not be taken into account. Major constituents of the virgin polyamide membrane included in descending order S, Na and Cl. Although other elements such as Ti, P, Table 4 Components of the virgin and fouling layer deposited on samples 1 and 2 obtained by XRF semi-quantitative analysis. Concentration (wt.%) Element Virgin Element Sample 1 Sample 2 S Na Cl Ti P Ca Si Ni Mn Al V 17.600 3.560 1.110 0.489 0.046 0.027 0.009 0.008 0.005 0.004 0.001 S Si Al Cl Na Ti F K Mg Fe Ca P Cr Mn Ni V Total 22.86 13.900 4.220 2.290 2.220 0.974 0.585 0.352 0.300 0.253 0.195 0.064 0.039 0.015 0.012 0.007 0.004 25.43 14.200 3.120 1.780 3.830 1.080 0.604 0.245 0.261 0.383 0.138 0.059 0.031 0.000 0.011 0.008 0.000 25.75 50 N. Melián-Martel et al. / Desalination 305 (2012) 44–53 Ca, Si, Ni, Mn, Al and V, were present in the composition of the polyamide, their contribution was not significant (b 1%). The higher percentage of Na in the virgin membrane is related to the detection of Na compound as standard preservation solutions. Filmtec elements are preserved in a standard storage solution containing a buffered 1 wt.% food-grade sodium metabisulfite (SMBS) [36] which prevents biological growth during storage and shipping of elements. In contrast, Cl is absent in the chemical structure of polyamide thin-film composite membranes [37]. However, in this study, the presence of Cl could be associated to the formation of the thin selective layer. In many commercial practices, the thin selective layer is formed by interfacial polymerization of amine monomers brought into contact with aromatic acid chloride monomers [38]. Therefore, Cl ions could have diffused through the polyamide skin layer with some retained in the microporous support. Sulfur was the element having the highest concentration in the fouled membranes likely due to the polysulfone support layer of the membrane [39]. Si and Al were the second and third highest concentration of metallic elements present in the fouled membrane. The size of these clay particles in feed seawater is typically less than 5 μm. Therefore, they pass through the micron filter and are trapped within the hollow fibers. Over a long period of operation, even less than 0.1 mg/l Al in the clays can accumulate to appreciable levels and contribute to highly insoluble aluminosilicate deposits. The next highest element contents were Cl followed by Na. A significant increase in the percentage of Cl in the fouled membrane was observed compared to the virgin membrane. Other minor elements such as Ti, followed by F, K, Mg, Fe, Ca and P with a concentration less than 2% in total and trace elements such as Cr, Mn, Ni and V with concentrations smaller than 0.04% in total were detected in the fouled membranes. The semi-quantitative results from the XRF confirm the results obtained through XRD analysis; specifically, the elements of Si, Al and Cl are contained in halite and in hydrogen aluminosilicate, which were identified as the prime deposits from the XRF analysis. To complement the results of the semi-quantitative XRF analysis and provide a better picture of the scale deposits a quantitative assessment by SEM-EDX, ICP-OES and IC was carried out. The X-ray analysis by SEM-EDX method was performed by means of area and spot analysis. The area analysis presents the average concentration of the elements distribution on the part of the membrane shown on the SEM image, while the spot analysis provides information of the chemical composition in that particular location. EDX results of the virgin membrane (Fig. 4b2) confirm the presence of the following elements in descending order C (52%) O (23%), S (11%), N (8%), Na (5%) and Cl (0.7%) which are in good accordance with those obtained by XRF. EDX spectra of foulant layer surfaces were very similar for most samples (Fig. 4a2). The analysis indicates that the deposits on the fouling layer had high levels of Si (8%), Al (5%) and Cl (5%), whereas the polyamide thin film matrix had high levels of O (46%), C (20%) and S (7%). Relatively low levels of Na (3%), Mg (2%), Fe (2%), K (1%), Ti (0.4%) and Ca (0.2%) were also present. The differences found between the semi-quantitative XRF and EDX data, are attributed to the fact that the fouling layer is unevenly distributed over the membrane surface. The higher concentration of the relation O/C in the fouled sample is likely due in part to organic and/or biological foulants. Particles with high content of Si, O and Al are frequently found all over the surface on the membrane, as shown in Fig. 6. The X-ray analysis described elsewhere [40] showed the major constituents of foulants to be Si, C, Al, and Fe, suggesting that the deposits contained significant amounts of organic material, silicate clay minerals, and iron compounds. The SEM-EDX investigation of the cross-section of the membrane gives further insights into the development of the fouling layer. The fouling layer presented in Fig. 7 has a thickness of less than 5 μm and consisted of particulate matter embedded in an amorphous matrix. It can be seen that the layer consisted of three distinct regions. Region 1 where no particulate matter was embedded within the inner amorphous layer and the EDX analysis (Fig. 7a) of this layer did not detect any elements except C, O and S. The same O/C ratio between region 1 and the virgin membrane support layer was found. Therefore, this region corresponds to the polysulfone support layer. Regions 2 and 3 were structurally and chemically different from region 1. The fouling layer shows a similar composition to those observed in the analysis of the top surface with the presence of Si, S and Al predominant and the presence of other compounds such as Fe, Na, Cl, and Mg with the highest concentration in region 3 (Fig. 7b and c). However, even between regions 2 and 3 the fouling patterns are quite different, with fewer elements detected in region 2. Sulfur was a major foulant in region 2, indicating that sulfur containing compounds were able to penetrate inside the membrane pores. The fouling layer has been produced by an accumulation of organic and inorganic substances, as the layer shows no clear continuity. The information obtained from the cross-section investigation provides insights into evolution deposition which are important in the development of a more complete understanding of the fouling mechanisms. Therefore, the elemental analysis throughout the cross-section of the membrane is important and can be used to discover which substances can penetrate inside the membrane pores and which substances mainly remain on the surface. The results of the chemical analysis by ICP-OES/AAS as well as the wavelengths used are summarized in Table 5 and the results of IC Fig. 6. SEM micrograph and associated EDX showing aluminum silicates scaling. 51 N. Melián-Martel et al. / Desalination 305 (2012) 44–53 Fig. 7. SEM micrograph of the cross-section and associated EDX analyses in different positions (a–c) of the fouled membrane. analysis in Table 6. The chemical analysis by IC confirms the detection of significant quantities of water soluble SO42−, Cl, Na +, Ca +2, Mg +2 and K +. The same pattern also was found by AAS, however, the concentrations of Na, Ca, Mg and K by AAS were higher than that obtained by IC. Therefore, the results confirm that the extraction of the deposits was more effective after digestion of the samples with 1:1 HCl than after leaching with HPW and ultrasound, as expected. The abundance of Na on the virgin membrane is attributed to the preservation coating on the membrane material. The content of Na + and Cl − is higher in the fouled membrane samples revealing significant deposits from the feed stream, thus confirming the results from XRD and XRF analysis. Calcium is one of the most common and important cations, and its complexation with natural organic matter such as humic substances results in a highly compacted fouling layer and flux decline has been long studied [41,42]. The next highest cation was K + followed by Mg 2+. The ICP-OES analysis of the fouled membrane revealed that other elements were also present (Cr, Zn, Ni, B and Mn) in small quantities. The most abundant of these were Cr, Zn and Ni that often appear in the composition of the aluminosilicates [14]. B is also present in the feed seawater. These results are in good accordance with those obtained by XRD. Tables 5 and 6 show an irregular pattern of the foulant concentration. Therefore, the foulant pattern on the RO surface was not formed in a compact layer. As seen in Fig. 1, the pre-treatment in the full scale desalination plant is a typical one involving chlorination for the destruction of bacteria, the addition of anti-scalants and filtration Table 5 Results of the chemical analysis of the virgin membrane and deposits by ICP-OES and AAS after leaching with HCl 1:1 from random samples of the fouled membranes. Concentration (mg/kg membrane) Elements/wavelength (nm) Virgin Elements/wavelength (nm) Sample 1 Sample 2 Sample 3 Sample 4 Analytical method Na (589.0) Ca (422.7) K (766.5) Mg (285.2) Fe (259.9) Al (396.2) 3571.4 463.8 225.9 212.1 111.2 12.8 Na (589.0) Ca (422.7) K (766.5) Mg (285.2) Al (396.2) Cu (324.8) Fe (259.9) Cr (283.6) Zn (213.9) Ni (231.6) B (249.7) Mn (257.6) 2510.6 4032.7 711.3 317.1 746.3 395.1 238.4 88.6 0.0 36.9 32.0 5.5 6212.7 4601.2 1042.0 1119.3 818.8 278.1 26.5 112.0 0.0 25.3 35.5 4.5 3672.7 2697.3 1014.6 723.7 624.8 274.5 148.8 311.4 154.2 66.4 0.0 1.7 4353.9 1303.6 1566.1 500.9 658.8 269.6 166.5 143.7 142.0 28.7 0.0 2.3 AAS AAS AAS AAS ICP-OES ICP-OES ICP‐OES ICP‐OES ICP‐OES ICP‐OES ICP‐OES ICP‐OES 52 N. Melián-Martel et al. / Desalination 305 (2012) 44–53 Table 6 Results of ion chromatography analysis after extraction with HPW from random samples of the fouled membranes. Ions 2− SO4 Cl− NO3− PO4 3− Na+ Ca2+ Mg2+ K+ Concentration (mg/kg membrane) Virgin Sample 1 Sample 2 Sample 3 Sample 4 21,217.3 2061.1 363.7 0.0 1658.0 233.2 77.7 181.3 1838.2 12,366.3 1002.7 3008.0 1893.6 562.9 255.9 255.9 351.1 2633.4 175.6 0.0 4685.9 617.9 823.9 489.2 705.4 3527.1 282.2 0.0 2181.7 128.3 179.7 154.0 506.1 1897.8 253.0 379.6 2774.7 192.3 247.3 247.3 – through sand filters and afterwards through cartridge filters. The formation of inorganic precipitates on the membrane surface may occur due to the following reasons: (a) The development of the biofilm on the membrane surface increases the selectivity of the membrane thus increasing the retention of other substances. (b) Cartridge size is quite large and does not retain the substances smaller than 5 μm. Probably smaller size cartridge filters (i.e. 1 μm) could retain more of the feed stream particles. (c) Operating pressure is increased in order to maintain constant the permeate quantity and this further exacerbates the fouling problem. – – 3.3. Evaluation of analytical methods As summarized in Table 7, based on the analytical methods employed for membrane fouling evaluation, the following can be noted: – Visual inspection and OM offers limited capabilities and only serve as a preliminary step in the evaluation of membrane fouling. It can be used to visually identify regions where significant fouling has occurred as opposed to other areas with much lower fouling. However, more powerful analytical methods must be employed to obtain information concerning the severity and type of fouling. – SEM-EDX is a valuable tool for studying surface structure and morphology. The level of resolution offered by SEM is important, as it can identify the pattern of membrane fouling. The elemental – analysis conducted to determine the fouling elements on the membrane can be used to identify the elements that mainly contribute to fouling. However, it offers semi-quantitative information and thus has to be used in combination with other analytical tools (i.e. ICP-OES) for the determination of foulants and to establish a whole picture of the fouling state. The same is true for the XRF analysis if only the semi quantitative mode is used. For the quantitative analysis by XRF appropriate membrane standards have to be used. ATR-FTIR is a powerful tool that can be employed to determine the presence of specific functional groups that contribute to the fouling layer. However, the presence of a large number of different foulants adds to the difficulty of obtaining consistent results concerning the importance of different functional groups with respect to RO fouling. This technique represents an attractive option for quality screening because it is rapid and inexpensive. As research on membrane fouling is focusing more on the impact of specific substances rather than on the simplistic examination of the impact of organic and inorganic substances or suspended and dissolved solids, analytical tools which identify the presence of specific compounds are gaining acceptance. IC provides valuable quantitative results concerning the main water cations and anions that contribute to membrane fouling. By means of AAS the main fouling elements of the RO membrane were determined; while by ICP-OES it was possible to analyze the trace elements too existing in the investigated membranes. However, this technique is influenced by the means used to extract the foulants from the membrane into the solution. Therefore, for consistency the same procedure must be used for the extraction process (i.e. chemical type and concentration, digestion duration and temperature). No analytical technique can be used on its own to assess all the different types of fouling. Rather, a combination of analytical methods is required to assess inorganic and organic fouling. A combination of ATR-FTIR for the determination of organic foulants, XRD, IC for inorganic foulants, ICP-OES/AAS for main and trace elements and SEM for visual representation can provide a comprehensive assessment and reliable characterization of the different types of foulants and better decision-making for fouling control. It should be mentioned that all the analytical techniques used, except from ICP-OES/AAS and IC, are non destructive methods and the samples are analyzed fast as is without any pretreatment. Table 7 Summary table of techniques used for structural and chemical characterization of fouled RO membranes. Technique Sample preparation Capabilities Advantages Limitations OM No preparation procedure is required No preparation procedure is required Fast Only basic-preliminary information Taking image with high resolution. Able to image and monitor elements in fouling layer Rapid and inexpensive Differentiating chemical composition of organic compounds in membrane foulants via fingerprint analysis Monitoring the average content of inorganic compounds in foulants Identifies specific inorganic elements Complementary to EDX Semi-quantitative information Only the surface of the sample can be monitored Mostly qualitative information ATR-FTIR No preparation procedure is required XRD No preparation procedure is required No preparation procedure is required Limited Identification of regions 2 D imaging of surface and cross section structure and morphology of fouling Characterizing substances in foulants Characterizing functional groups that contribute to the fouling layer Depth profiling. Indirectly information on biofouling Characterizing inorganic substances in foulants Characterizing inorganic elements in foulants Leaching with suprapure 1:1 HCl (~75 °C) Leaching with suprapure 1:1 HCl (~75 °C) Characterizing the main inorganic and trace elements in foulants Characterizing the main inorganic elements Leaching with suprapure H2O in an ultrasonic bath Cations and anions that contribute to membrane fouling SEM-EDX XRF ICP-OES AAS IC Quantitative analysis Quantitative results Quantitative results Semi-quantitative analysis. Only the surface of the sample can be monitored Semi-quantitative analysis It is limited to dissolved samples It is limited to dissolved samples. Higher detection limits than ICP-OES Limited number of cations and anions N. Melián-Martel et al. / Desalination 305 (2012) 44–53 4. Conclusion The chemical and structural data obtained for the fouled membranes are of a great importance in order to understand the nature and mechanisms of membrane fouling in RO systems and extremely useful in setting up effective membrane cleaning protocols and improving pre-treatment design and operation, so as to mitigate membrane fouling. The main results obtained from different techniques are consistent and complementary to each other, leading to the following conclusions: – The examination by OM and SEM showed that the fouling layer consisted of particulate matter embedded in an apparently amorphous matrix which was unevenly distributed over the membrane surface with the greatest accumulation in the valley areas. – Inorganic foulants analyzed by XRD consisted mainly of hydrogen aluminosilicates and halite, showing a significant amount of colloidal fouling on the membrane. – Proteins and polysaccharides were identified by ATR-FTIR as important foulants and were probably the result of biofouling. – The chemical analysis by ICP-OES, XRF, IC and SEM‐EDX showed that Si, Al, Cl, Ca, Na, Mg and K were by far the predominant constituents of the deposits. Other minor trace elements were Ti, Zn, Fe, Cu, Cr, Mn and Ni. – The information obtained from the cross-section investigation provides insights into evolution deposition of foulants as reflected in the differences in composition analyzed by SEM-EDX. – The prominent reasons for the deposition of foulants on the RO membranes are the increase of membrane selectivity due to biofouling, the large size of the cartridge filters and the high operating pressure. – The appropriate use of the analytical methods presented in this work for the chemical and structural characterization of RO membrane fouling can improve our understanding with respect to RO fouling and can lead to ameliorating the RO membrane performance and reducing the operating costs of RO plants. Most of the methods used are non destructive. – The analytical tool of ATR-FTIR is expected to be more frequently used in the future for the evaluation of specific substances that impact on membrane fouling. 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