Fate and Transport of Nanomaterials in Aquatic Environments - Nanotechnologies for Water Environment Applications - page 541

interactions occurring between the hexagonally arrayed carbon atoms in the graphite
sheet of MWCNTs and the aromatic backbones of the dyes. Biocompatibility
experiments performed in vitro and in vivo showed that caged MWNTs had higher
cell density than cells cultured without the caged MWNTs or caged activated carbon.
The caged MWNTs may adsorb the essential factors, such as the trace amount of
growth factors, for cell growth (Fugetsu et al., 2004).
(2) NMs can enhance or inhibit the mobility of chemicals sorbed to or passing through
NMs. In air, aerosolized NPs can absorb gaseous or particulate pollutants. Materials
sorbed to the gallium oxide increase retention in the respiratory tract, and increase
exposure to the stomach, liver, and kidney (Sun et al., 1984; Chen et al., 2000). In
soil or sediments, NMs might increase the bioavailability of pollutants, thereby
increasing the pollutant's availability for biodegradation (USEPA, 2007).
Tungittiplakorn et al. (2005) reported that NPs made of an amphiphilic polymer
mobilized phenanthrolene from contaminated sandy soil and increased its
bioavailability. In soil or sediments, nano-Ceo or CNTs could either enhance or
inhibit the mobility of organic pollutants (Cheng et al., 2004). Stable colloids of
hydrophobic NMs in an aqueous environment could provide a hydrophobic
microenvironment that suspends hydrophobic contaminants and retards their rate of
deposition onto soil and sediments (USEPA, 2007). However, NOM (e.g., humic
acids) may coat onto NMs as discrete colloids (e.g., 1-5 nm, Gibson et al., 2007),
and therefore, change the surface characteristics of the NMs, making NMs more
mobile and facilitate the transport of other pollutants (colloid-facilitated transport)
(Kretzschmar and Sticher, 1997).

Recently, membranes made of CNTs were used for water treatment (Kalra et
al., 2003; Hinds et al., 2004; Srivastava et al., 2004; Majumder et al., 2005; Holt et
al., 2006). The gas and water permeabilities of these CNT-based membranes
(diameter = 2-7 nm) were found several orders of magnitude higher than those of
commercial polycarbonate membranes (diameter =15 nm). This is because slip
boundary conditions allow a larger flux (Holt et al., 2006) as indicated by the Hagen-
Poiseuille equation with a slip flow correction (Holt et al., 2006):
Q = sji[(d/2) 4 + 4(d/2) 3 LJAp/(8p,TL)

(Eq. 15.97)

where Q = the volumetric flow rate; Ap = the pressure drop; d = the pore diameter; e
= relative porosity; p, = the water viscosity; T = tortuosity; L s = the slip length; and L
= the membrane thickness. Ls can be calculated as follows (Holt et al., 2006):

L s = LW(dU/dr)

(Eq. 15.98)

where U wa ii is the axial velocity at the wall, and dU/dr is the radial velocity gradient
at the wall (or shear rate); Ls can also be found with (Lauga et al., 2005)