Elsevier

Desalination

Volume 333, Issue 1, 15 January 2014, Pages 118-125
Desalination

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

https://doi.org/10.1016/j.desal.2013.11.033Get rights and content

Highlights

  • 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.

Abstract

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.

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], [2], [3], [4], [5], [6], [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], [9], [10], [11], [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 cross flow velocity) and foulant characteristics [13], [14], [15], [16], [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], [19], [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], [24], [25], [26], [27]. In addition, these analyses provide information about the type and 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], [29], [30], [31], [32], [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], [38], [39], [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.

Section snippets

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

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

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

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