Environmental Engineering Reference
In-Depth Information
mWCNTs with two different types of amino acids: lysine and arginine. mWCNTs functionalized with both amino acids were
found to enhance the interaction of mWCNTs with
Salmonella typhimurium
,
E. coli
, and
S. aureus.
The positively charged
arginine and lysine groups improved the adsorption of bacterial cells in mWCNTs, increasing the effectiveness of antimicrobial
activity. Qi et al. [81] performed another modification by immobilizing mWCNTs with cefalexin and investigated the contact
of mWCNTs with Gram-positive bacteria (
S. aureus
and
B. subtilis
) and Gram-negative bacteria (
E. coli
and
Pseudomonas
aeruginosa
). The result shows that the antimicrobial activity of the produced nanocomposite is more effective in Gram-positive
bacteria because cefalexin mainly inhibits peptidoglycon synthesis.
8.5
Cnt-based membranes
The excellent physicochemical properties of CNTs are attracting attention because of their possible application in membrane
technologies. A recent discovery on fast water transportation in the inner hollow cavity of CNTs also increases the potential of
CNTs as a material for water treatment. Extremely rapid water transport is about two to five times higher than the value pre-
dicted through the Hagen-Poiseuille equation [82, 83]. Theoretical studies using molecular dynamic simulation have shown
that water can fill the empty inner cavity of CNTs within a few tens of picoseconds. The weak interaction of water molecules
with the hydrophobic walls of CNTs enables frictionless transport and high water flux [84]. Detailed studies on the transport
mechanism have been conducted because of the rapid water transport to the nanoscale confinement in the CNT channel.
Confining water inside the CNT channel narrows the interaction energy distribution, eventually lowering the chemical potential
and free energy [85]. The confinement also induces stronger attraction between water molecules than the interaction with CNT
walls [86], which promotes a higher water flux.
The beneficial effect of CNTs in water transport can be exploited in the development of membrane-separation technologies.
Their presence is a massive breakthrough in membrane technologies and provides a synergistic effect for water treatments.
CNT-based membranes are versatile materials for various membrane-separation applications including ultrafiltration [87-89],
nanofiltration [90-93], reverse osmosis [94-96], and membrane distillation (mD) [97-101]. Studies have shown that CNT-
based membranes can overcome the limitation of conventional membranes. The use of CNT-based membranes can also address
cost-effectiveness issues.
CNTs may have different macroscopic structures and arrangements in membranes depending on the synthesis method. The
most common strategy is the incorporation of CNTs as fillers in the polymer matrix by blending them with polymer solution to
form so-called CNT-mixed matrix membranes (CNT-mmms) [102]. CNTs may also appear in bundle forms arranged like
paper sheets, which are known as buckypapers. They may also be uniformly aligned perpendicular to the membrane surface,
which forms vertically aligned CNT membranes.
8.5.1
Cnt-mmms
Figure 8.3 shows that CNTs are randomly oriented inside the polymer matrix of CNT-mmms. The presence of a small amount
of CNTs can improve the properties and generate new characteristics in the resultant membranes. Shaw et al. [94] synthesized
CNT-mmms composed of mWCNTs and polyamide for use in water treatment. CNTs are well recognized for their ability to
sustain high energy loading. Thus, CNTs serve as reinforcing agents that can improve the Young's modulus, toughness, and
tensile strength of CNT-mmms. However, ensuring adequate interfacial bonding between the polymer matrix and CNTs is
necessary to elicit the maximum reinforcing effect of CNTs. Hence, CNTs are usually modified with functional molecules prior
to blending with polymer.
CNT-mmms also enhance the chemical resistance of polymeric membranes particularly against chlorine, which is com-
monly found in water [95, 104]. Polymeric reverse osmosis membranes such as polyamides are susceptible to degradation
upon contact with chlorine, which in turn causes deteriorated membrane-separation performance. Polyamide degradation is
mainly attributed to the chlorination of N-H in amide bonds upon exposure to chlorine, followed by oxidation to the quinoid
structure through hydrolysis reaction with water. The deformation of an amide network causes serious defects in the mem-
brane structure, thereby decreasing salt rejection [104]. The presence of CNTs effectively increases the chemical resistance of
polyamide membranes against chlorine. Experimental results showed that salt rejection in CNT-mmms surpasses that in
conventional polyamide membranes after exposure to high-concentration chlorine solution. For polyamide membranes, salt
rejection decreased by 21.8% after chlorination. However, addition of 0.1, 0.5, and 1% (w/v) CNT-mmms decreases salt
rejection by 15.8, 11.2, and 10.1%, respectively. The interaction between carboxylic groups in acid-modified CNTs and the
amide bond in polyamide is believed to inhibit the chlorination of amide bond, which enhances the chlorine resistance of
CNT-based membranes [95].
