Nanotechnology for domestic water purification - Nanotechnology in Eco-efficient Construction - page 408

Holt et al. , 2006), can help in the formulation of the next generation of
membranes with improved fl ux, slectivity and anti-fouling. The fast mass
transport of water through CNT would ensure high fl ux (Majumder et al. ,
2005a). The functionalization of the CNT tip, as shown in Fig. 16.14, would
confi rm the gate-keeper controlled chemical separation (Majumder et al. ,
2005b). In addition to the foregoing, the CNT itself or appropriately func-
tionalized CNTs would make the membrane surface anti-biofouling (Kang
et al. , 2008; Narayan et al. , 2005). However, there are numerous challenges
associated with each step of membrane making, starting from growth of
CNTs to membrane performance evaluation and scale up. Primarily, there
are four approaches (Majumdar and Ajayan, 2010) to the synthesis of mem-
branes based on CNTs as shown in Fig. 16.15.

1. Deposition of carbonaceous materials inside pre-existing ordered
porous membranes, such as anodized alumina, also known as the tem-
plate synthesized CNT membranes (Miller et al. , 2001).
2. Membranes based on the interstice between nanotubes in a vertical
array of CNTs, subsequently referred to as the dense-array outer-wall
CNT membrane (Srivastava et al. , 2004).

3. Encapsulation of as-grown vertically aligned CNTs by a space-fi lling

inert polymer or ceramic matrix followed by opening up of the CNT
tips using plasma chemistry, or the open-ended CNT membrane (Hinds
et al. , 2004; Holt et al. , 2006).

4. Membranes composed of nanotubes as fi llers in a polymer matrix, also

known as mixed-matrix membranes.

Although CNT-based membranes possess excellent properties to emerge
as next generation membranes, still a signifi cant number of challenges
remain to be tackled. To grow 12-13 order of magnitude CNTs per square
centimetre is a real technological challenge, though chemical vapour deposi-
tion methodology offers excellent parameters to achieve this objective. It
is very diffi cult to obtain the yield of CNTs in a particular batch of synthesis
beyond 90% reproducibly. Tedious purifi cation steps to remove the sooty
deposits and to make the CNT wall defect free make things more compli-
cated. The nanocomposite membrane fabrication route has to confi rm that
CNTs are well dispersed and well aligned, which is highly challenging. It
may require functionalization of CNTs to have better dispersion. Function-
alization of CNTs with desired functional groups requires substantial
knowledge of Chemistry, for CNT is not soluble in any solvent. The most
critical step, that is opening of the CNT tips with either acid treatment or
plasma-based oxidation, is not that trivially simple to be adopted. Moreover,
the tip-opening step may cause thinning of the CNT wall and disruption of
tube integrity and subsequent failure of membrane channels. Finally, scale-
up with respect to CNT growth, CNT alignment, nanocomposite formation,

￿ ￿ ￿ ￿ ￿ ￿