Environmental Engineering Reference
In-Depth Information
and water through the membrane. They found that the gas and water permeabilities of
these CNT membranes are several orders of magnitude higher than those of commercial
polycarbonate membranes (diameter = 15 nm), despite having pore sizes of an order of
magnitude smaller. Water flux was 10
3
times the flux predicted by H-P equation. L
s
was10
3
times the nanotube diameter, while L
s
= 5 nm for a polycarbonate membrane (d
= 15 nm). Thus, CNT membranes can be a new separation technique with great potential
for applications in reduction/treatment of wastes and water purification.
It should be noted that there are different opinions on transport mechanisms of
NF and CNT membranes, such as, based on pressure difference over the membrane,
concentration gradient across the membrane, electrostatic repulsion between the charged
membrane, non-slip boundary along the CNTs. Retention of organic/inorganic
compounds is determined by membrane properties (e.g., molecular weight cut-off,
membrane charges, properties of the gatekeepers at the entrance/tips of CNT
membranes, among others), compound properties (e.g., molecular size and charges,
hydrophobicity, ionization constant, etc.), and operating conditions of the membrane
system (Bellona et al., 2004; Majumder et al., 2005a,b; Smuleac et al., 2006).
14.3.5 Nanomaterials for Groundwater Remediation
Nanoparticles are widely used to remove certain classes of organic pollutants
from groundwater, especially those that are not easily biodegradable, and can be injected
into subsurface groundwater to remediate contaminated plumes (Gnaser et al., 2004).
Due to their size distribution and certain surface characteristics, nanomaterials can travel
to contaminated zones and can be carried to great distances with groundwater. For
example, nano-sized iron particles are very effective for the transformation and
detoxification of a wide variety of common environmental contaminants, such as
chlorinated organic solvents, organochlorine pesticides, and PCBs (Joo and Cheng,
2006). Modified iron nano-particles, such as catalyzed and supported nano-particles
have been synthesized to further enhance the speed and efficiency of remediation. In
addition, nano-particles can be impregnated onto a solid matrix such as carbon, zeolite,
or membrane for enhanced treatment of water, wastewater, or gaseous process streams.
14.4.1 Nitrate Removal from Water
Only a small fraction (4.9%) of the earth's surface water is fresh and suitable for
human consumption. 50% of the world's population depends on groundwater for
drinking water. Among several N species, nitrate (NO
3
-
) contamination of groundwater
is a major concern throughout intensive agricultural areas (i.e., it is a NPS pollution).
Search WWH ::
Custom Search