Fate and Transport of Nanomaterials in Aquatic Environments - Nanotechnologies for Water Environment Applications - page 538

show essentially irreversible absorption and, thus, are more suitable as a delivery system
for ground water remediation (Velegol and Tilton, 2001; Braem et al., 2003; Saleh et al.,
2005; 2007; Krajangpan et al., 2008).
Ditsch et al. (2005) studied the clustering and stability of magnetic NPs coated
with random copolymers of acrylic acid, styrenesulfonic acid, and vinylsulfonic acid.
Clusters larger than 50 nm formed when the coatings were made using too low or too
high molecular weight polymers, or when insufficient amounts of polymer were used.
Low-molecular-weight polymers resulted in thin coatings that did not sufficiently screen
van der Waals attractive forces, while high-molecular-weight polymers bridged between
particles. Insufficient polymer amounts resulted in incomplete coverage and the
formation of bare patches on the magnetite surface. For incompletely coated magnetite
NPs, the stability of the resulting clusters was poor, but if the coverage of the primary
coating is inadequate, a secondary polymer could be added to coat the remaining bare
magnetite substrate to improve control over aggregation. With properly controlled
aggregation, the clusters are stable in high salt concentrations (> 5M NaCl), while
retaining the necessary cluster size for efficient magnetic recovery.
Many studies have been focused on modifying the particle size and properties of
NMs. He and Zhao (2007) reported that the particle size of NZVI synthesized by
borohydride reduction of iron sulfate using carboxymethyl cellulose (CMC) as a
stabilizer could be altered by changing the CMC/Fe + molar ratio. While NZVI with a
hydrodynamic diameter of 18.6 nm could be synthesized at Fe concentration of O.lg/L
and with 0.2% (w/w) of CMC, smaller NZVI NPs could be obtained by increasing the
CMC/Fe + molar ratio. Moreover, CMC with a greater molecular weight or higher
degree of substitution, and lower synthesizing temperature were found favoring the
formation of smaller NZVI.
Saleh et al. (2005, 2007) have shown that amphiphilic triblock copolymers with
an A-B-C triblock micro structure are effective delivery systems to enhance NZVI
transport and DNAPL targeting ability in groundwater remediation. They used
amphiphilic p(methacrylic acid)-p(methyl methacrylate)-p(styrene sulfonate) (PMAA42-
PMMA26-PSS462) to decorate NZVI surfaces; the architecture of the blocks in the co-
polymer was as anchoring-hydrophobic-hydrophilic. The amphiphilic triblock
copolymer enhanced the stability and affinity of NZVI NPs for a water/organic interface.
However, kinetic studies showed a decrease in the rate of contaminant degradation by
the polymer-modified NZVI as compared to unmodified NZVI (Saleh et al., 2007). The
reduction in the contaminant degradation rate was attributed to low permeability of the
contaminant through the film adsorbed onto the NZVI. Saleh (2008) proposed to
optimize DNAPL targeting ability of NZVI by synthesizing amphiphilic block
copolymers in three different architectures, that is, hydrophobe as terminal block, a
gradient of hydrophobicity in the chain, and randomized hydrophobes in random