Environmental Engineering Reference
In-Depth Information
approaches, the post-synthesis modification of the nanoparticles has been investigated
most extensively. Some studies related to surface modifications are briefly reviewed
below.
Size is one of the most important parameters that control the dispersion
characteristics of particles (Logan, 1999). Colloid transport through soils is easiest for
particles having sizes less than 50 nm and greater than 3 nm (Logan, 1999). However,
the Tufenkji-Elimelech model (Tufenkji and Elimelech, 2004), which considers the
effect of hydrodynamic forces and van der Waals interactions between the colloidal
particles and soil/sediment grains, predicts that the optimal particle size for effective
flow of nZVI in groundwater is between 200 and 1000 nm (Hydutsky et al., 2007;
Phenrat et al., 2007). Particles smaller than 200 nm undergo collisions with aquifer
particles (soil grains) more frequently than their larger counterparts because the smaller
particles are subjected to Brownian motion (Hydutsky et al., 2007). Phenrat et al. (2007)
reported that iron nanoparticles behave as a ferromagnet once the size exceeds 15 nm
which leads to aggregation of the nanoparticles.
Bare nZVI do not disperse in aqueous media and are susceptible to excessive
oxidation by non-target compounds (Giri et al., 2001; He and Zhao, 2005; Krajangpan et
al., 2008). As a result, they are not suitable for groundwater remediation. Nanoparticle
surface modification and/or functionalization are essential for enabling effective
groundwater remediation using nZVI. The motivation behind most efforts to surface
modify nanoparticles is to increase their injectability into the subsurface and increase
their reactivity towards contaminants. Surface modification is expected to inhibit
agglomeration, and thereby, maximize the reactive surface area for contaminant
remediation. Surface modification and functionalization are widely practiced on
nanoparticles used for biomedical applications. Different types of modifiers have been
used to modify nanoparticle surfaces for biomedical purposes, such as carboxylate-
functional compounds (Sahoo et al., 2005), phosphate-functional compounds (Portet et
al., 2001; Mutin et al., 2003), silica (Johnson et al., 1996; Alcala and Real, 2006), gold
(Lin et al., 2001), dextran (Lee et al., 2002), polyethylene glycol (Kim et al., 2001; Paul
et al., 2004), polyvinyl alcohol (Nishio et al., 2004), alginate (Kroll et al., 1996), and
chitosan (Lee et al., 2005).
To avoid agglomeration, surfactants or polymers are often used to modify
nanoparticle surfaces during or after the synthesis of the particles. Papell (1965)
invented the ferrofluids which are now used to increase the colloidal stability of
magnetic nanoparticles. Water or oil-based ferrofluids are commercially available, and
they are usually stable at pH < 5 (acidic ferrofluid) or pH > 8 (alkaline ferrofluid). By
controlling the surface charge and using specific surfactants, stability of ferrofluids can
be enhanced (Lu et al., 2007). Kim et al. (2005) coated iron oxide nanoparticles (~15
nm) with oleic acid before dispersing them in chitosan biopolymer to make ferrofluids.
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