Nanotechnology for domestic water purification - Nanotechnology in Eco-efficient Construction - page 376

adsorption isotherms are well described by both the Langmuir and
Freundlich models.
The fi rst data on multi-wall nanotubes (MWNTs) as sorbent for dioxin
removal was reported by Long and Yang (2001). A technique based on
temperature-programmed desorption (TPD) was used to study dioxin
adsorption. The amount adsorbed on CNTs is 1034 higher than on activated
carbon. Hence, signifi cantly higher dioxin removal effi ciency is expected
with CNTs than with activated carbon. The strong interaction between
dioxin and CNTs may be attributed to the unique structure and electronic
properties of CNTs. The CNTs consist of hexagonal arrays of carbon atoms
in graphene sheets that surround the tube axis. Strong interactions between
the two benzene rings of dioxin and the surface of the CNTs are expected.
Srivastava et al. (2004) reported the fabrication of freestanding macro-
scopic hollow cylinders having radially aligned CNT (ACNT) walls, with
diameters and lengths up to several centimetres. These cylindrical mem-
branes are used as fi lters in the elimination of multiple components of
heavy hydrocarbons from petroleum - a crucial step in post-distillation of
crude oil - with a single-step fi ltering process, and the fi ltration of bacterial
contaminants such as Escherichia coli or the nanometre-sized poliovirus
( 25 nm) from water. These macro fi lters can be cleaned for repeated fi ltra-

tion through ultrasonication and autoclaving.
Techniques have been developed to synthesize CNTs in signifi cant quan-
tities using three different methods:

∼

1. arc discharge,
2.

laser ablation and

3. chemical vapour deposition (CVD).

An overview of these different routes is summarized in Table 16.2
(Balasubramanian and Burghard, 2005).
Of the various methods of CNT synthesis, CVD has got tremendous
potential for scaleup. In addition, the growth of CNTs can take place without
having patterned substrate (with impregnation of catalyst particles). A
special case of CVD is called spray pyrolysis, where liquid precursors like
mixture of benzene and ferrocene can be used. The experimental setup is
shown in Fig. 16.5 (Dasgupta et al. , 2008). It consists of a sprayer, a container
for the liquid precursor and a quartz tube (inner diameter 10 mm). The
sprayer is made up of a pyrex nozzle (inner diameter 0.4 mm) and an
outer pyrex tube with an exit diameter of 2 mm. The inner nozzle
carries the liquid precursor and the outer one carries the nitrogen gas. The
sprayer is attached to the quartz tube kept inside a resistive furnace. An
optimized combination of composition and the fl ow rate of liquid precursor
solution, the carrier gas fl ow rate, the diameter of inner and outer nozzles
of sprayer and the temperature of growth helps in the synthesis of CNTs

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