Page 335 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

320 Environmental Applications of Nanomaterials

Velegol and Tilton 2001). Electrostatic, steric, or electrosteric repul-
sions decrease nanoparticle interactions with mineral grains (Saleh
et al. 2007) and potentially also with soil bacteria, which may decrease
the observed bactericidal properties of nanoparticles. For example,
Goodman et al. (2004) found that gold nanoparticles with positively
charged side chains were toxic to E. coli, but negatively charged par-
ticles were not. Rose et al. (2005) found that positively charged CeO 2
nanoparticles adsorbed to E. coli and were bactericidal, showing a
clear dose-response.
Poly(methacrylic acid)-b-poly(methylmethacrylate)-b-poly(styrene
sulfonate) triblock copolymers (PMAA-PMMA-PSS), PSS homopolymer
polyelectrolyte, sodium dodecylbenzene sulfonate (SDBS), polyethyl-
ene glycol (PEG), carboxymethyl cellulose (CMC), and guar gum have
been shown to adsorb to and stabilize dispersions of nanoiron and Fe-
oxide nanoparticles to improve particle mobility in the environment
due to steric, electrosteric, or electrostatic repulsions between the par-
ticles and between the particles and soil grains (He and Zhao 2005;
Saleh et al. 2007). Poly(aspartic acid), an anionic polypeptide, also
adsorbs to iron oxide surfaces via acid-base interactions through the
carboxyl groups (Chibowski and Wisniewska 2002; Drzymala and
Fuerstenau 1987; Nakamae et al. 1989) and can be an effective stabi-
lizer for metal-oxide nanoparticles. Alkyl polyglucosides are an emerg-
ing class of surfactant synthesized from renewable raw materials and
are nontoxic, biocompatible, and biodegradable. They adsorb to metal
oxide surfaces (Matsson et al. 2004). They are desirable due to their
low cost and “green” nature and widely used as detergents in manu-
facturing. A systematic investigation of the ability of these types of
surface modifiers to enhance nanoiron mobility by minimizing particle-
particle interactions as well as particle-media grain interactions is
underway.
Polyelectrolytes have been used to provide functionality to nanoiron
(e.g., affinity for the DNAPL/water interface [Saleh et al. 2005a]) as well
as to enhance their transport in laboratory columns. For example,
transport of triblock copolymer-modified nanoiron is significantly
enhanced in a 10-cm sand column relative to the unmodified nanoiron,
especially at the high nanoiron concentration that is needed to be
cost-effective (Figure 8.12) (Saleh et al. 2007). In this case, enhanced
transport is due to an adsorbed layer of a strong polyelectrolyte that
provides electrosteric repulsions, which limit the particle-particle inter-
actions as well as the particle sand-grain interactions. Enhanced mobil-
ity comes at a price, however, as modified particles were four to nine
times less reactive with TCE than for unmodified particles (Saleh et al.
2007). Despite the lower reactivity, the particles are sufficiently reac-
tive with TCE to be effective in situ groundwater remediation agents.