The effect of gel layer thickness on the salt rejection performance of polyelectrolyte gel-filled nanofiltration membranes

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Abstract

The effect of gel layer thickness on salt separation of positively charged pore-filled nanofiltration membranes has been examined both theoretically and experimentally. The extended Nernst-Planck (ENP) equation coupled with the Teorell-Meyer-Sievers (TMS) model were used to calculate the pressure-driven sodium chloride rejections for membranes having gel densities in the range typically used in nanofiltration applications. It was found that salt rejection was dependent on membrane (gel-layer) thickness with salt rejections increasing rapidly with thickness up to 50–75 μm. Further increases in thickness beyond this point had a much smaller effect on salt rejection. The theoretical predictions were examined experimentally by preparing a series of membranes with cross-linked poly(3-acrylamidopropyl)-trimethylammonium chloride (PAPTAC) gels with varying densities within the pores of a thin microporous polyethylene (PE) support. The membranes were characterized by their polymer volume fractions (gel concentration), thicknesses and effective charge densities. The effect of membrane thickness was examined by using single and stacks of two membranes. The pure water fluxes and salt rejections of the membranes and membrane stacks were determined in the pressure range 50–550 kPa. The single salt rejections of the membranes which were very dependent on the thickness of the membrane or membrane stack, were fully in accord with the calculated salt rejections of the membranes.

Introduction

Polyelectrolyte gel-filled nanofiltration (NF) membranes are an attractive alternative to the conventional thin film composite (TFC) membranes for various applications, including low pressure water softening [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. These gel-filled membranes consist of a polyelectrolyte gel anchored within a microporous support. The porous support restricts osmotic swelling of the gel and provides mechanical strength as well as dimensional stability. The separation properties are determined by the gel component. With charged gels ionic solutes are rejected predominantly on the basis of the Donnan exclusion mechanism. The underlying factors that control the performance of polyelectrolyte gel-filled (both positively and negatively charged) membranes for nanofiltration applications have been reviewed [22].

A model has been developed to calculate the hydrodynamic permeability of gel-filled membranes from first principles [20]. In terms of ion rejection the extended Nernst-Planck (ENP) equation combined with the Teorell-Meyer-Seivers (TMS) model can be used to predict the pressure-driven ionic rejections of charged gel-filled membranes [23]. A steric component to salt rejection is only important when the gel–polymer volume fraction is relatively high [24].

Polyelectrolyte gel-filled NF membranes differ from other NF membranes in terms of the thickness of the separating layer. In conventional NF membranes the separating layer is typically thin and relatively dense. On the other hand, in the gel-filled membranes the separating layer is a thick but relatively low density polyelectrolyte gel. Typically the gels are evenly distributed throughout the thickness of the supporting MF membranes and the resulting thickness of the separating layer is that of the support plus any osmotic swelling that occurs due to the anchored gel. Most gel-filled NF membranes that have been studied have thicknesses which range from ca. 125 to 200 μm. It is interesting that despite the comparatively large thickness of the separating layer the gel-filled membranes can have fluxes which match those of the more conventional thin film NF membranes [21], [22].

Clearly by making the gel layers in gel-filled membranes thinner it would be possible to increase their fluxes and, potentially, greatly improve their performance. Reduction of the gel layer thickness could be done by either using thinner MF supporting membranes or creating a membrane with an asymmetric gel distribution through the thickness of a MF support [26]. The question that arises is how does the thickness of the gel-layer in a polyelectrolyte gel-filled membrane affect its separation properties? Two approaches are used in this work to answer this question. The first involves a theoretical approach using the extended Nernst Planck equation coupled with the Teorell-Meyer-Sievers model. The second uses an experimental approach involving a series of thin membranes containing poly(3-acrylamidopropyl)trimethylammonium chloride (PAPTAC) gels. The performance of these membranes was studied as single or as stacks of membranes. The results of both approaches gave consistent findings and allowed for the definition of an optimal gel thickness for water softening NF membranes.

Section snippets

Theoretical background

An analysis of the salt rejection properties of gel-filled membranes was initially reported using a Peclet number approach [19]. More recently Mika and Childs have reported an analysis of salt separation by positively charged gel-filled membranes in terms of the TMS model for charged membranes combined with the Manning-Oosawa counterion condensation theory to estimate their effective charge density [23]. The advantage of this approach is that it was possible to predict the separation properties

Materials

The porous support material used in this study was a poly(ethylene) microfiltration membrane supplied by 3M Company that was prepared by thermally induced phase separation process (TIPS) [37], [38]. The membranes had a porosity of 76 vol% and a thickness of 20 ± 2 μm.

Surfactant Triton® X-114 and monomer (3-acrylamidopropyl) trimethyl ammonium chloride (APTAC, 75 wt% solution in water), and N,N′-methylenebisacrylamide (MBAA) were obtained from Aldrich. The photoinitiator 2-hydroxy-1-[4-(2-hydroxy

Results and discussion

The strategy used in this work was to model the effect of membrane thickness for a series of membranes filled with different polymer volume fractions of a polyelectrolyte and to compare the findings with experimental results. PAPTAC gels were selected for this study as experimentally they show good performance and can be readily made with good control over the polymer volume fractions [40]. The range of polymer volume fractions studied are those typically used for polyelectrolyte gel-filled NF

Conclusions

The model calculations as well as experimental results described in this article have shown that thickness plays an important role in the performance of gel filled membranes. The calculation of salt separation based on the extended Nernst Planck equation coupled with the TMS model showed that this effect is more pronounced for thinner membranes, particularly at low fluxes. No significant improvement in separation was observed at thickness >75 μm and as any increase in thickness is costly in

Acknowledgements

This research was supported by National Sciences and Engineering Research Council and 3M Canada Company.

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