Page 378 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Membrane Processes 363

Polymer-TiO nanoparticle composites have been created with the
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objective of creating antifouling membranes. These membranes would
exploit the photocatalytic properties of TiO to produce hydroxyl radi-
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cals that would then, in turn, oxidize organic foulants depositing on the
membrane surface. Membranes decorated with TiO nanoparticles have
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been fabricated in both UF [48] and RO [49] formats. These membranes
particles at functional sites
are formed by self-assembly of the TiO 2
(such as sulfone or carboxylate groups) on the membrane surface.
Alternatively, the TiO nanoparticles can be immobilized within the
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membrane matrix by introducing these materials as a mix during the
process of membrane casting [50]. While both formats (decorated and
immobilized) appear to mitigate fouling by bacterial growth, the deco-
rated format appears to be more effective due to the higher amount of
TiO on the membrane surface [50].
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An inherent conflict in membrane design must be overcome in devel-
oping a photocatalytic system for reducing membrane fouling. The eco-
nomics of membrane module design dictate that a maximum of
membrane area be contained within a given volume (high packing den-
sity). However, this design objective is in conflict with the objective of
providing adequate illumination of the membrane surface to promote
photocatalysis. There may be niche applications for photocatalytic mem-
branes where a high packing density is not required to make the process
economically feasible.
Membranes may also be modified by depositing particles on the
surface for the purpose of simultaneous catalytic degradation, sensing,
or simply improved selectivity. Nanoparticles with molecular imprints
of a specific compound can be created and then attached to conven-
tional membranes to impart a high specificity of separation for the
imprinted molecule [51]. Catalytic nanoparticles (such as TiO or iron)
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can be attached to membrane surfaces with the objective of degrading
some compounds while physically separating others. However, the
objective of creating a so-called reactive membrane is fraught with
conflicts in the efficiency of the overall system. Reaction rate consid-
erations tend to favor a membrane with a slow permeation rate (high
residence time) to allow for sufficient conversion of the desired com-
pound(s) as the fluid they are contained within flows across the mem-
brane. On the other hand, the efficiency in fluid separation pushes the
system toward a higher permeation rate and shorter residence time.
The conflict between reaction rate and residence time is resolved for
the case of reactive membranes that work on materials sorbed to the
surface of the membrane. This might be the case for either the destruc-
tion of sorbed foulants or the accumulation and subsequent destruction
of a contaminant for which the membrane is designed to have an
enhanced adsorptive affinity.