Overview of Nanoscience in the Environment - Environmental and Human Health Impacts of Nanotechnology - page 19

1.13.3 Fate in Soil

The potential fate and behaviour of engineered nanomaterials in soil can be under-
stood in the light of the existing knowledge on the fate and behaviour of natural
colloidal particles (Chapter 4). Nanomaterials are small enough to travel through
soil pores. However, they can be sorbed to soil particles due to their high surface
area and, therefore, become immobilized. In addition, the formation of large aggre-
gates of nanomaterials can immobilize them by fi ltration, sedimentation or strain-
ing in smaller pores. At the moment, little information is available on the transport
and fate of nanomaterials in the natural porous environment. However, some data
are available from laboratory column studies using porous media (Lecoanet et al. ,
2004; Lecoanet and Wiesner, 2004; Li et al. , 2006 ; Schrick et al. , 2004; Yang et al. ,
2007b), which suggest that transport is often relatively rapid and dependent on the
type of nanomaterials.
Laboratory soil column experiments on iron oxide and zero-valent iron nanopar-
ticles show that their mobility is more limited due to the effi cient fi ltration mecha-
nisms of aquifer material. Field studies on iron oxide nanoparticles indicate that
they may migrate only few centimetres to few metres from the point of injection
and that their mobility is dependent on many factors, such as particle size, solution
pH, ionic strength, soil composition and ground water fl ow velocity (Li et al. , 2006 ;
Schrick et al. , 2004). The zero-valent nanoparticles are somewhat more mobile as
they have been synthesized on supports acting as a delivery vehicle (Schrick et al. ,
2004; Yang et al. , 2007) These delivery vehicles, including anionic hydrophilic
carbon and poly(acrylic acid) (PAA), bind strongly to the iron, create highly nega-
tive surfaces, thus effectively reducing the aggregation among zero-valent iron
particles, and reduce the fi ltration removal by aquifer materials. Laboratory soil
column experiments with iron/hydrophilic carbon, iron/PAA and unsupported iron
nanoparticles suggest that the anionic surface charges can enhance the transport
of iron nanoparticles through soil and sand packed columns in comparison with
unsupported iron nanoparticles (Schrick et al. , 2004 ; Yang et al. , 2007 ). In addition,
the transport of iron nanoparticles (2-10 nm) through a porous media column (glass
beads, unbaked sand and baked sand) can be enhanced by surface modifi cation via
surfactant sorption. Un-modifi ed iron nanoparticles were immobile and aggregated
on porous media surfaces in the column inlet area (Kanel et al. , 2007 ). Although
surfactants and polymers enhance the transport of nanoparticles, the role of natural
organic matter in nanoparticle facilitated transport has not yet been investigated,
but is likely to be important. Further, the characteristics of the soil matrix may
infl uence the diffusion and transport of nanoparticles. PAA-modifi ed nanoiron
slurry has been found to travel easily through silica sand columns, but not loamy
sand soil columns (Yang et al. , 2007 ).
The transport of water stable ' nC 60 particles ' underivatized C 60 crystalline
nanoparticles, stable in water for months through a soil column was investigated at
different fl ow rates, while other column operating parameters remained fi xed
through all the experiments. The nC 60 particles were observed to be more mobile
at higher fl ow velocity due to less interaction time between the nC 60 particles and
the porous media (Cheng et al. , 2005). Lecoanet and Wiesner (2004) studied the