Environmental Engineering Reference
In-Depth Information
groundwater because of the scarcity of clean surface water. However, groundwater often
contains dangerous levels of arsenic, luoride, and other contaminants.
1
In modern times, water sources are being progressively contaminated primarily due to
industrialization and rapid increase in population. It is impossible to imagine the survival
of the world without potable water. Contamination of water sources is broadly of two
types, natural (geogenic) and anthropogenic. Water bodies have been polluted largely due
to anthropogenic activities compared with natural sources. Toxins in water are chemical
and biological. Chemical toxins include pesticides (chlorpyrifos [CP], malathion, endosul-
fan, atrazine, etc.), halocarbons (CCl
4
, C
6
H
5
CH
2
Cl, CH
2
Cl
2
, etc.), dyes (cationic and anionic),
heavy metal ions (Hg
2+
, Pb
2+
, Cd
2+
, etc.), anions (F
−
,
AsO
3−
,
AsO
3−
, CN
−
,
NO
3
−
, etc.), and
organic molecules such as drugs and pharmaceuticals. Bacteria, viruses, fungi and algae
are important biological pollutants. The presence of any one of these components in water
makes it unsafe for drinking. Consumption of water containing the above species causes
various diseases and even death. As a result, there is a great demand for technologies
for clean drinking water. Important toxins of water, their permissible limits, origins, and
health effects are listed in Table 26.1.
In view of the immense requirement of clean water, there is a large need of technologies
for purifying water. The available methods for water puriication are adsorption, disinfec-
tion, coagulation, locculation, sedimentation, membrane iltration, and reverse osmosis.
There are advantages and disadvantages associated with each method. For any method to
be successful, it should be economically viable and must have reduced or no power con-
sumption and preferably have features such as easy operation, facile synthesis of materi-
als, recyclability, need for less manpower, and capacity to purify water in large quantities.
The maximum allowed contamination levels in water are decreasing as time progresses.
For example, permissible limits of arsenic and lead decreased from 200 to 10 ppb and 100
to 10 ppb, respectively (according to World Health Organization [WHO] 1990 standards).
The US Environmental Protection Agency (US EPA) also decreased permissible levels of
lindane, arsenic, and lead from 4.0 to 0.2 ppb, 50 to 10 ppb, and 50 to 15 ppb, respectively,
during the period 1976-2001. From these data, it is clear that technologies have to be devel-
oped for sensing and removal of pollutants at ultra-low levels.
2
Technologies involving
materials with high surface area and varieties of reactive adsorption sites are advanta-
geous. This may be possible when materials are at the nanoscale (10
−9
m).
Nanomaterials, materials with physical dimensions (diameter, thickness, grain size)
<100 nm in length, are shown to be excellent candidates for the puriication of water. The
reasons are their large surface-to-volume ratio, unusual reactivity, as well as size-dependent
optical, physical, and chemical properties. Many nanosystems have been explored by
researchers in the recent past in the context of treatment of water. Some of them are
nanoscale carbon (graphene, graphene oxide [GO], reduced GO [RGO], GO/RGO-metal/
metal oxide composites, and carbon nanotubes), nanosized zerovalent iron, and oxides of
aluminum, iron, titanium, magnesium, cerium, and manganese.
3
Among metals, noble
metals are also used for decontamination of water. Use of noble metals, although not in
nano form, for this application, has been documented since ancient times. People stored
drinking water in containers made up of metals (copper/silver) or clay without knowing
the exact reasons. Later, researchers found the inactivation of bacteria and other biologi-
cal entities in these vessels. Studies showed that these metals were actually damaging the
DNA of bacteria.
Noble metals are those metals with high resistance to aerial oxidation/corrosion even
at high temperatures. Nobility varies from one metal to the other. Most of them are less
abundant in the earth's crust, because of which, they are precious. Examples of noble
