Biomedical Engineering Reference
In-Depth Information
Terrestrial ecosystems are composed of soil and organisms. Soil primarily consists of air, water,
organic matter, and minerals (Brady and Weil 1996). NMs move around within the pore space in
soil and interact with organic matter and minerals. The fate and transport of NMs in the soil is also
difficult to predict, as it depends on the physical and chemical properties of the NMs as well as the
soil (Darlington et al. 2009, Doshi et al. 2008, Jaisi and Elimelech 2009, Saleh et al. 2008). These
properties may lead to aggregation, adsorption, absorption, dissolution, stabilization, transport,
or deposition. For example, electrostatic interactions have been observed between the negatively
charged, citrate gold NP and positively charged particles in soil, leading to the attachment of NMs
on soil particles. Soil solution chemistry parameters, such as ionic strength, pH, and the presence
of organic matter, strongly affect the interactions of NMs with solid media; this influences the
balance between the free migration of particles and the deposition of NMs (Solovitch et al. 2010).
Dissolved organic matter in the soil interacts with NMs and can alter their fate, transport, and
bioavailability in the soil. Owing to their small size, NMs have the capability of traveling deeper
through soil pores and may get trapped within the soil matrix (Brar et al. 2010). Also, organic
molecules such as humic and fulvic acids present in the soil can stabilize NMs in soil solutions and
may further enhance their abilities to travel longer distances within the soil (Jaisi and Elimelech
2009). This can ultimately lead to the transport of these NMs to underground water systems. In a
study by Yang et al. (2009), it was found that humic acids were adsorbed on the surface of TiO
2
,
Al
2
O
3
, and ZnO NPs, leading to a decreased zeta potential. This indicated that humic acid-coated
NPs of metal oxides can be easily dispersed and suspended in solution because of enhanced elec-
trostatic repulsions. Organic matter adsorbed on NMs also reduces their aggregation, which may
influence their movement in soil solutions.
Interactions of NMs with the water between pore spaces is of great importance, as it directly
affects the plant roots and hyphae, such as in the case of fungi (Navarro et al. 2008). NMs can
also interact with pollutants in the soils. These pollutants can be organic, such as pesticides, or
inorganic, such as metal oxides. The interaction of NMs with these pollutants can further alter
the fate and behavior of NMs within the soil medium. Soil colloids with high surface areas carry
absorbed minerals and other soil particles. Therefore, they play a major role in the transport of
pollutants within the soil medium (Wilson et al. 2008). Only a few publications have documented
the uptake of NMs by living organisms within the soil. Fabrega et al. (2009) found that the inter-
action of bacteria with NMs can affect the transport of NMs in soil. There is evidence to support
that NMs can be transported from the soil to plants. Kurepa et al. (2010) found evidence that
modified TiO
2
can enter plants cells and accumulate in certain subcellular locations. Another
study (Lin and Xing 2008) observed the root uptake and toxicity of ZnO NPs in various plants.
It was found that in the presence of ZnO NPs, biomass production was significantly reduced,
root tips shrank, and the root epidermal and cortical cells were highly damaged. Some evidence
also suggests that NMs can spread via terrestrial plants (Lin et al. 2009). The ecological impact
and behavior of NMs on the entire terrestrial ecosystem remains underreported. It is imperative
to assess the impact of NMs on soil and terrestrial plants as well as how much they leach to the
underground water system.
In an aquatic environment, the small size and large surface area of NMs make them important
binding phases for other organic and inorganic contaminants. Other properties such as high surface
energy, quantum confinement, and conformational behavior are also considered important. The
association and stabilization of NMs with natural organic material, as well as with other organic
contaminants, is relevant in order to study their toxicological implications in aquatic ecosystems.
The main plausible causes of engineered NM contamination in aquatic systems are waste water
treatment plants, production facilities, industrial processes, accidents during transport, and inten-
tional releases. Once in the environment, free NPs tend to form aggregates that can be trapped
or eliminated through sedimentation (Figure 1.11). These trapped aggregates can be taken up by
organisms that feed on sediment. Although this may potentially lead to distortions in the food chain,
no data are currently available on this topic.
