Environmental Engineering Reference
In-Depth Information
the high-speed electrical sterilization of water using silver nanowires, CNTs, and cotton
(Figure 12.13a through g). 70 This approach used a gravity fed device that can inactivate
>98% of bacteria with only several seconds of total incubation time with a moderate bias of
20 V (Figure 12.13h). The device works by the use of an electrical mechanism rather than
size exclusion, while the very high surface area of the device coupled with large electric
ield concentrations near the silver nanowire tips allows for effective bacterial inactivation.
The CNT component of the system ensures good electrical conductivity over the entire
active area of the device, so it can be placed at a controlled electric potential and used in
solution as a porous electrode. The ultimate impact of this technology may be direct imple-
mentation as a cheap POU water ilter for deactivating pathogens in water, or perhaps
more probably as a new component to be integrated into existing iltration systems to kill
microorganisms that cause biofouling in downstream ilters. 70
CNT technology has potential advantages over conventional disinfection approaches
and has great application in water disinfection. Currently, the cost of CNTs stands as a
major roadblock preventing them to consider for large-scale applications such as water
treatment. Release of CNTs into the environment and human exposure are other major
concerns at this point. Hydrothermal treatment of carbohydrates yield functionalized car-
bon nanomaterials that have found extensive applications in catalysis, drug delivery, etc. 71
These carbohydrate-derived carbon nanomaterials can be synthesized at very low cost and
form different nanostructures such as carbon nanospheres and nanotubes/nanocables. 71
These carbon nanomaterials and their inorganic hybrids (with Ag, TiO 2 , etc.) can be prom-
ising alternatives for CNTs in water disinfection owing to their inexpensive synthesis.
12.3.4 Clay-Inorganic Composites
Most of the mesoporous materials used in the water ilter applications are based on ceram-
ics, clay, sand, silicates, etc. Ceramic or sand-based materials are ideal for the development
of water treatment systems for the developing countries, particularly for water disinfec-
tion. Tap water seeded with different microorganisms or untreated wastewater was passed
through columns containing sand modiied by the in situ precipitation of metallic hydrox-
ides or unmodiied sand. Columns packed with 1 kg of sand modiied with a combination
of ferric and aluminum hydroxide removed >99% of E. coli, Vibrio cholerae , poliovirus 1,
and coliphage MS-2 from dechlorinated tap water. 72 This removal eficiency was consistent
throughout the passage of 120 liters of water during a 30-day test. After the passage of 192
liters of tap water, these columns were still able to remove 99.9% MS-2, 80% E. coli , and
90% of poliovirus. Columns containing modiied sand eficiently removed microorganism
from water samples at the various pH and temperature (>4 log 10 reduction at room tem-
perature) values tested, while columns containing unmodiied sand did not. In addition,
these modiied sand ilters were able to remove coliform bacteria and coliphage from raw
sewage. The modiied sand seems to better remove microorganisms because of increased
electrostatic interactions. Neither E. coli nor MS-2 was inactivated by the modiied sand
column efluents. The metal used for coating the sand could not be detected in the column
efluents, indicating that the coatings were stable. 72
To make these clay or silicate materials to be used for eficient water disinfection appli-
cations, they are also impregnated with antibacterial agents. In most of the cases, it is sil-
ver that is used for impregnation and silver-ceramic water ilters are used extensively in
water disinfection. Oyandel-Craver and Smith manufactured cylindrical colloidal-silver-
impregnated ceramic ilters for household (POU) water treatment and tested for per-
formance in the laboratory with respect to low rate and bacteria transport. 73 The ilters
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