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Alumina Nanofibre Membranes for Filtration and Scaffolding

Introduction

Nanofibre assemblies, having a structure with considerable mechanical strength, have good potential in membrane filtration. They possess several advantageous features due to the use of nano and sub-micron size ranges of fibres, including high specific surface area, light weight and higher molecular orientation, which in turn could be modulated via fibre diameter, surface characteristics and structural quality of nanofibers [1]. As a separation process, membrane filtration is potentially used for removal of solvent (concentration), ions and small molecules (desalination) and particles (clarification) [2]. Ceramic based nanofibres membranes can be produced by depositing nanofibres from aqueous dispersion on a mesh support, followed by compression, drying & sintering of the fibrous network [3]. The nanoscale alumina fibres (NAF) supplied by Metallurg Engineering (Estonia), are all single crystals of γ-alumina having a diameter of the order of 10-20 nm (Fig.1). These are produced by controlled oxidation of a molten bath of aluminium in large quantities (kg h-1 range), thus making them attractive for membrane production.

Fig.1 (a) Photograph of as-grown slab, (b) SEM micrograph, showing individual fibre alignment, and (c) TEM micrograph of a single fibre.

 

Production of NAF Membranes

Membranes have been produced from fibres of this type by dispersion in aqueous suspension, followed by sedimentation onto a coarse mesh. This is illustrated in Fig.2. An impression of the fibre architecture [1] in typical membranes is given by Fig.3. By controlling the stirring process, structures ranging from fully dispersed fibres to assemblies of fibre bundles can be obtained.

 Fig.2  Schematic representation of membrane production by sedimentation of nanofibres from aqueous liquids.


Fig.3 SEM micrographs of membranes with (a) fully dispersed (homogeneous) structure and (b) partially dispersed (micro-bundled) structures.

 

Measurement of Permeability

For effective filtration, a combination is required of a fine scale structure (to promote entrapment of filtered species) and a relatively high permeability (to promote rapid throughput and minimise clogging of the filter). The specific permeability, κ, of these membranes was measured using the set-up in Fig.4(a). As shown in Fig.4(b), the presence of bundles can raise the permeability, while retaining fine filtration capacity.


 

Fig.4 (a) schematic of the set-up for permeability measurement and (b) depiction of the effect of bundles (multi-scale structures) on flow patterns.

 

Membrane Permeability

Experimental data are shown in Fig.5 for the permeability of two membranes, produced after short and long periods of dispersion before sedimentation. It can be seen that there was a lower fibre content, and a higher permeability, after the shorter period, due to the presence of fibre bundles. It can also be seen that the permeability is higher in the direction of sedimentation. Also shown are predictions from an equation based solely on the fibre content and surface area.

 

 Fig.5 Experimental data, and Carman-Kozeny model predictions, for the permeability of membranes after two different periods of stirring.


Filtration Performance

It has been shown that excellent filtration can be obtained, depending on network architecture, introduction of additional species (such as silica gel) into the interstices between fibres and also charge-related effects [4]. For example, Fig.6 shows that relatively small dye molecules can be separated, depending on their charge.

 

 Fig.6 Beakers containing aqueous solutions of (a) Methylene Blue, (b) Methyl Orange, a mixture of MB and MO and (d) filtrate on passing (c) through a silicated bundle membrane.

 

Membranes as Scaffolds

These membranes offer promise as scaffolds for the support of fine particles or coatings. For example, there is interest in the photo-catalytic effect of fine titania partcles in breaking down impurities in water, using either UV or visible light. Fig.7 shows membranes after passage of an aqueous suspension of 20 nm TiO2 particles (with a coating of silver, which improves their photocatalytic function). It can be seen that these particles do become entrapped in these membranes. The plot demonstrates that these particles strongly absorb visible and UV light.
Fig.7 Membranes (on the left) after use to filter suspensions of Ag-coated TiO2 nanoparticles. The top membrane was fully-dispersed and silicated and the bottom one is made up of bundles. The plot (on the right) shows absorption of light through the initial suspension and the filtrate.


Conclusions

  1. An alumina nanofibre has been used to create fine scale membranes, which have strong potential for offering attractive combinations of ultra-filtration and relatively high permeability.
  2.  A promising area of application for these membranes is as a scaffold for the support of fine particulate with photocatalytic properties. Preliminary work has confirmed that such particles can be entrapped.

Acknowledgements

This work has been supported by IDB-Cambridge Commonwealth, European and International Trust, Metallurg Engineering and Trinity Hall. Thanks to M. Coto and Dr R.V. Kumar (The Materials Chemistry Group) for the TiO2 particles.

References

[1]          H. Matsumoto and A. Tanioka, "Functionality in Electrospun Nanofibrous Membranes Based on Fiber’s Size, Surface Area, and Molecular Orientation," Membranes, vol. 1, pp. 249-264, 2011.

[2]          X. Ke, Y. Huang, T. R. Dargaville, Y. Fan, Z. Cui, and H. Zhu, "Modified alumina nanofiber membranes for protein separation," Separation and Purification Technology, vol. 120, pp. 239-244, 2013.

[3]          V. Su, M. Terehov, and T. W. Clyne, "Filtration Performance of Membranes Produced Using Nanoscale Alumina Fibers (NAF)," Advanced Engineering Materials, vol. 14, pp. 1088-1096, 2012.

[4]          V. Su and T. W. Clyne, "Hybrid filtration membranes incorporating nanoporous silica within a nanoscale alumina fibre scaffold," Advanced Engineering Materials, in press, 2015.

 

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