Environmental Engineering Reference
In-Depth Information
knowledge on the fate and behaviour of natural colloidal particles (Chapter 4). This
knowledge suggests that the fate of nanomaterials in the aquatic environment can
be infl uenced by a variety of processes, such as: dispersion/diffusion, aggrega-
tion and disaggregation, interaction between nanoparticles and natural water
components, sedimentation, biotic and abiotic degradation, transformation and
photoreaction/light. These reactions may alter the physical and chemical properties
of nanomaterials and so alter their behaviour in the aquatic environment.
There have been few studies on the aqueous stability and aggregation of
nanomaterials under environmentally relevant conditions. Brant et al. (2005)
studied the aggregation and deposition of fullerene nanoparticles in aqueous media
at variable ionic strength. They found that, while in the absence of electrolytes nC 60
stayed stable over time, 0.001 M solution ionic strength (NaCl) was enough to
destabilize the nC60 by screening their electrostatic charge and produce large
aggregates that settle rapidly (Brant et al. , 2005). The addition of humic acid has
been shown to enhance the stability of fullerene suspension in the presence of
sodium chloride and magnesium chloride and low concentrations of calcium choride
(Chen and Elimelech, 2006). However, at high concentrations (above 10 mM) of
calcium choride, enhanced aggregation of fullerene nanoparticles was observed due
to bridging mechanism by humic acid aggregates (Chen and Elimelech, 2007).
Other research has found similar complex interactions between natural and
manufactured nanomaterials (Baalousha et al. , 2008a ; Giasuddin et al. , 2007 ;
Hyung et al. , 2007 ).
Extracted Suwannee River humic acid (SRHA) and natural surface water (actual
Suwannee river water with unaltered natural organic macromolecules (NOM)
background) has been shown to stabilize multi-wall carbon nanotubes (MWCNTs)
(Hyung et al. , 2007 ). However, extensive fl occulation of multi-wall carbon nano-
tubes (i.e. formation of fl oating aggregates and partial sedimentation of other
aggregates) was observed when mixed with natural waters from a lake, presumably
due to the high ionic strength and the presence of divalent cations such as calcium
(Baalousha et al. , 2008a). Apparently, sorption of humic substances enhances the
stability and inhibits the aggregation of carbon nanotubes to a certain extent
(Hyung et al. , 2007), However, cations, particularly divalent cations such as calcium
and magnesium reduce the stability of carbon nanotubes in the absence or presence
of natural organic matter surface coating (Baalousha et al. , 2008a ).
Disaggregation is as important as aggregation in determining the fate and behav-
iour of nanomaterials, though few studies are available (Baalousha, 2009; Ouali and
Pefferkorn, 1994). Natural organic matter has been shown to induce the disaggrega-
tion of iron oxide nanoparticle aggregates (5- 10
m) formed at pH 7, likely due to
formation of a surface coating of natural organic matter on the surface and pore
surface of the aggregates and thus the enhancement of surface charge (Baalousha,
2009). In addition, it has been shown that certain polymers are able to disaggregate
latex particle (885 nm in diameter) aggregates (Ouali and Pefferkorn, 1994).
However, polysaccharide or humic acid did not result in the disaggregation of
polystyrene latex particle aggregates; this was explained by the existence of strong
interparticle forces within fl ocs which prohibited aggregate break-up upon adsorp-
tion of natural organic matter (Walker and Bob, 2001).
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