Environmental Risks of Nanomaterials - Nanotechnologies for Water Environment Applications - page 608

17.4.4 Minimizing Environmental Exposure and Hazard
The higher mobility of NPs in the environment suggests that they are dispersed
over greater distances and their effective exposure persists in the environment. To
reduce exposure and hazard potential, NMs can be attached to surfaces or to form
aggregates. For example, NMs that are attached to mineral surfaces may be less mobile
in porous media, such as subsurface or the sand filters used in water treatment.
Deposition and aggregation of NPs can be described as a two-step process. They are
different in that particle deposition involves attachment to an immobile site, whereas
particle aggregation involves attachment between mobile sites. NM deposition in an
aquifer can be described as a sequence of the particle transport to the immobile surface,
followed by attachment to the collector ( O'Melia, 1980 ).
It is essential to understand the transport behavior of NMs in natural waters to
predict the fate in the environment before we decide what to do for controlling their
transport and thus minimizing the exposure. As the physicochemical properties of NMs
are caused by their high surface-to-mass ratio, understanding the aggregation behavior
of NPs in the environment is essential. In the aquatic environment, the NPs tend to
aggregate and therefore their transport in the environment is likely restricted to
sedimentation processes (Brant et al., 2005). Parameters such as pH, ion types and
concentrations as well as humic substances can influence the surface properties and
chemical reaction of NPs. A simplified aqueous test matrix (SATM) can be applied, the
principles of which can be transferred to complex natural aquatic conditions. Such
results show that, at different NP concentrations, aggregation behavior of TiO 2 is
strongly dependent on pH and ionic strength in the solution. The presence of cation and
anion species and humic acids may significantly affect the stability of TiO 2 suspensions.
While approaches for characterizing and quantifying NPs remain largely undeveloped,
all potential sources of emission need to be identified, including those available during
production, handling, and use. Control technologies should be developed to ensure that
emissions are minimized. Respiratory deposition of NPs needs to be understood better.
17.4.5 Legislation for NMs
Legislation and guidance notes can affect the manufacture, use, and disposal of
NMs. Some of the major areas of legislation in Europe are listed as follows: Council
directive 67/548/EEC, Council regulations (EC) 793/93, the REACH legislation
(Registration, Evaluation, Authorisation and Restriction of Chemicals), Directive
24/1998 (to protect workers from chemicals, see also (Aitken et al., 2004), integrated
pollution prevention and control directives and EU council directive 96/61/EC (relating
to pollution prevention), and EU council directive 1999/31/EC (relating to waste
disposal). The Scientific Committee on Emerging and Newly Identified Health Risks
(SCENIRH) (European Commission, 2002) believes that NMs are not specifically