Page 342 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials for Groundwater Remediation 327

three months. It was stabilized by RNIP particles that had been modi-
fied by irreversible adsorption of PMAA -PMMA -PSS 466 followed by
42
26
several cycles of centrifugation and washing. For comparison, TCE and
water could not be emulsified by unmodified RNIP particles, proving
that the polymer was essential for anchorage at the interface. Likewise,
no emulsion was formed using the supernatant of a centrifuged polymer-
modified RNIP sample, proving that the interface was stabilized by
adsorption of polymer-modified RNIP, and not by adsorption of free
polymer that may have been present in solution with the nanoiron.
Furthermore, RNIP modified by PSS homopolymers was unable to emul-
sify TCE and water. This demonstrates that the type and composition
of the surface modifiers are critical design variables for interfacial tar-
geting.
There are many other approaches that could be used to achieve sig-
nificant targeting of contaminants. For example, enzymes-based
approaches may be possible. Enzymes are proteins or conjugated proteins
produced by living organisms and functioning as biochemical catalysts.
They typically have complex active sites that are highly selective for
specific compounds. This selectivity may be leveraged to tailor reactive
nanoparticles for specific compounds. Enzyme-coated carbon nanotubes
have been used as single molecule biosensors (Besteman et al. 2003),
and nanoparticles containing a single enzyme protected by a porous
inorganic/organic network have been created (Kim and Grate 2003).
These approaches may eventually be used to develop groundwater reme-
diation agents with the highest specificity for target compounds possi-
ble. Less specific approaches include ethylenediaminetetraacetic acid
(EDTA) coatings on TiO nanoparticles designed to sequester radionu-
2
clides (Mattigod et al. 2005). EDTAis a strong chelating agent that forms
coordination compounds with most divalent (or trivalent) metal ions,
2+ 2+ 2+
such as calcium (Ca ) and magnesium (Mg ) or copper (Cu ), and thus
can be used to sequester cationic groundwater contaminants such as
2+ 2+
Cu or Pb . Once strongly sequestered, these toxic metals are no longer
bioavailable and pose less or no risk to biota. Anatase (TiO ) nanoparti-
2
2+
cles coated with EDTAand treated with Cu to form an EDTA/Cu(II) com-
plex have been used to then sequester anionic groundwater contaminants

such as pertechtinate (TcO ). It is proposed that the pertechtinate forms
4
a strong complex with the bound EDTA/Cu(II) on the TiO surface.
2
Another approach is to use hydrophobic nanoparticles designed to strongly
sequester hydrophobic contaminants such as polyaromatic hydrocarbons
(PAHs) or polychlorinated biphenyls (PCBs). These contaminants are
> 4) and strongly adsorb to soil and
typically very hydrophobic (log K OW
sediment. These hydrophobic nanoparticles are added to the PCB- or
PAH-contaminated soil or sediment, where the hydrophobic contami-
nants can strongly adsorb to them, after which, the nanoparticles are