Page 371 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

356 Environmental Applications of Nanomaterials

Membrane Fabrication Using Nanomaterials
The performance of membranes is intimately linked to the materials
they are made from. The composition of the membrane will determine
important properties such as rejection (selectivity), propensity to foul,
mechanical strength, and reactivity. Membrane composition may even
determine the element geometries that are (or are not) possible and, of
course, the cost of the membrane. It is therefore not surprising that the
creation of new nanomaterials opens the door for many new approaches
to fabricating and improving membranes. We will consider three cases.
In the first case, we will consider examples where nanomaterials are
used to make membranes. In the second case, we will examine enhance-
ments in existing membrane materials achieved through the introduc-
tion of nanomaterials to create new composites. Finally, we will take a
look at the use of nanomaterials to “mold” membranes in a process
known as nanomaterial templating.

Membranes made from nanomaterials
In some instances, membranes may be made (nearly) entirely of prod-
ucts of nanochemistry. The advantages of these materials may include
improved processing, due for example to the lower sintering tempera-
ture required when nanoparticles are used as precursors to ceramic
membranes. Or they may include include improved performance linked
directly with the properties of the materials.

Ceramic membranes derived from mineral nanoparticles. Mineral mem-
branes have been made from a variety of mineral nanoparticle precur-
sors. Commercially available ceramic membranes are typically made
from metal oxides such as Al 2 O 3, ZrO 2 , and TiO 2 [15]. However, mem-
branes can be made from many other nanomaterials ranging from gold
[16, 17] to SiO 2 [18]. In most cases nanoparticles are deposited on a sup-
port surface and then calcined to create the membrane. Processes differ
in the manner in which nanoparticle precursors are prepared. One
common procedure for producing nanoparticle precursors to these
membranes is to precipitate particles under controlled conditions creating
a suspension or sol of nanoparticles that are deposited on a surface and
dried to form a gel. This procedure is known as sol-gel.
Sol-gel involves a four-stage process: dispersion, gelation, drying, firing.
A stable liquid dispersion or sol of the colloidal ceramic precursor is
initially formed in a solvent with appropriate additives. In the case of
alumina membranes, this first step may be carried out with 2-butanol
or iso-propanol. By removing the alcohol, the polymerization of aluminum
monomers occurs leading to a precipitate. This material is acidified, typ-
ically using nitric acid, to produce a colloidal suspension. By controlling