Environmental Engineering Reference
In-Depth Information
The mean hydrodynamic diameter of their coated particles in the chitosan solution was
estimated to be 65 nm. The stabilized ferrofluids with different iron concentrations were
stable for 30 d without precipitation.
Surfactants have been used with a varying degree of success for colloidal
stabilization of nanoparticles (Rosen, 2002). The hydrophobic “tails” of the surfactants
physically absorb onto the nZVI surface while the hydrophilic “heads” inhibit
flocculation and allow for suspension in the aqueous medium. While surfactants enable
colloidal stability in water, the highly reversible nature of surfactant absorption limits its
application as an nZVI delivery system for groundwater decontamination since
desorption will be favored when the nanoparticles are transported through surfactant-
free groundwater. Schrick et al. (2004) used poly(acrylic acid) and anionic hydrophilic
carbon supported nZVI for the removal of chlorinated hydrocarbons (Schrick et al.,
2004, Saleh et al., 2005). The delivery vehicle was able to lower the aggregation and
sticking coefficient of nZVI. However, poly(acrylic acid) has limited application due to
reversible adsorption characteristics (Schrick et al., 2004; Saleh et al., 2005). Sun et al.
(2007) used polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A), a
biodegradable surfactant, as a dispersant for nZVI. The addition of PV3A led to
significant enhancements in particle stability and subsurface mobility of nZVI. Other
effects of application of the surfactant included reduction of the mean particle size from
105 nm to 15 nm, reduction of the zeta (
)-potential from +20 mV to 80 mV at neutral
pH, and a shift of the isoelectric point (IEP) from pH ~8.1 to 4.5. They also found that
PV3A-stabilized iron nanoparticles were capable of effectively decomposing
trichloroethene (TCE). Bare nZVI (prepared without PV3A) with a median diameter of
59.4 nm and a mean diameter of 105.7 nm settled in less than 1 min. However, no
sedimentation of the PV3A-stabilized nZVI with a median diameter of 7.9 nm and a
mean diameter of 15.5 nm was observed for over a 6-month period (Sun et al., 2007).
These researchers also screened a number of commercially available polymers,
copolymers and surfactants including ionic and nonionic molecules. They tested
polyvinyl chloride, polyacrylic acid, polydimethylsiloxane, and polypropylene oxide as
possible nanoparticle surface modifiers. PV3A with a molecular weight of 4300-4400
proved to be the best. PV3A is of food grade, nontoxic, and biodegradable because of
the presence of -OH, -CO-, and -COOH groups. Song et al. (2008) stabilized CeO
2
-
coated SiO
2
nanoparticles with the anionic surfactant, sodium dodecylbenzene sulfonate
(SDBS), and the nonionic surfactant, polyethylene glycol (PEG). The dispersion
characteristics observed under different conditions indicated better dispersion of surface-
modified nanoparticles as compared to unmodified analogs. PEG is hydrophilic, and the
hydrated film developed around the PEG molecule gives rise to steric stabilization (Song
et al., 2008).
He and Zhao (2005) reported on the use of water-soluble starch for the
stabilization of palladized iron (Fe-Pd) nanoparticles. The modified nanoparticles were
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