Nanotechnology for domestic water purification - Nanotechnology in Eco-efficient Construction - page 364

industrialization. Coupled to this is the alarming issue of global warming
and climate change which is potentially responsible for uneven distribution
of rainfall. All these factors in turn lead to increasing levels of contamina-
tion and, moreover, newer types of contamination than encountered previ-
ously. The permissible limits of contaminants in a proposed safe drinking
water are becoming less and less with the passage of time (e.g., the recom-
mended maximum permissible value for arsenic in drinking water accord-
ing to WHO international standards has been reduced from 200 ppb to
10 ppb through a number of revisions in the last 50 years. The case is the
same with lead, where the limits have been brought down to 10 ppb from
50 ppb).
Given the deteriorating water resource situation, it is inevitable that
newer, more effi cient and more selective water purifi cation technologies are
required to take care of the specifi c contaminants at a very low level. The
technology to remove these contaminants should also reach molecular
limits so that capture can be highly effi cient in a minimum residence time.
On the other hand, such a process should be environmentally friendly,
economically feasible and with minimum or no use of electricity for rural
adaptability.
'Nanotechnology' has a tremendous role to play in such a precarious situ-
ation, where the reactions can take place at ionic/atomic/molecular scale in
a very selective manner with amazingly high effi ciency. Because of the sig-
nifi cant increase in surface-to-volume ratio in a nano-dimension, the con-
taminant uptake capacity becomes many fold. Compared to the conventional
water treatment technologies such as membrane-based treatment, activated
carbon, UV-based fi ltration, electrodialysis and distillation, the nano-based
systems would provide the following advantages (Pradeep and Anshup,
2009a):

￿ higher effi ciency of removal even at low concentration of adsorbents
￿

￿ ￿ ￿ ￿ ￿ ￿

functionalization capability of nanomaterials leads to specifi c uptake

￿ low waste generation.

Two of the major distinctions that defi ne types of conventional remedia-
tion technologies also apply to nanotechnologies for remediation: adsorp-
tive versus reactive and in situ versus ex situ . Absorptive remediation
technologies remove contaminants (especially metals) by sequestration,
whereas reactive technologies affect degradation of contaminants, some-
times all the way to harmless products (e.g., CO 2 and H 2 O in the case of
organic contaminants). In situ technologies involve treatment of contami-
nants in place, whereas ex situ refers to treatment after removing the
contaminated material to a more convenient location (e.g., pumping
contaminated groundwater to the surface and treatment above ground). In
situ degradation of contaminants, when feasible, is often preferred over