Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 145

liquid-gas interface can act as barrier for colloid movement if the water fi lm in the
unsaturated media is thinner than the diameter of the colloids. Colloids can also
be retained at the menisci of pendular rings of the fl uid-gas collector interface
instead of the liquid-gas interface (Crist et al. , 2004 ). Consequently, calculations
using equations from the CFT for an unsaturated soil system should be regarded
with care and can serve, if at all, only as a fi rst rough estimation. Also, preferential
fl ow in unsaturated soils, that is all phenomena where the fl uid move along certain
pathways while bypassing a fraction of the unsaturated porous media, has to be
included in transport predictions. Even though a lot of progress on preferential fl ow
has been made recently (Clothier et al. , 2008), prediction of colloid transport is
extremely diffi cult.
Colloid mobilization in natural unsaturated porous media is highly dependent
on transient fl ow conditions (Laegdsmand et al. , 1999). Summarized by Lenhart and
Saiers (2002), the transport of colloids in the unsaturated zone can be described as
advection and dispersion, together with a sink-source term. Advection- dispersion
is relatively well understood, but size exclusion (i.e. limiting the accessibility of
colloids to parts of the unsaturated pore space) and especially the sink- source term
(i.e. deposition and mobilisation) is less understood and an area of active research
(Crist et al. , 2004 ).

4.8 Conclusion

This chapter has reviewed colloids in natural aquatic and terrestrial systems and
primarily their aggregation, sedimentation and transport behaviour. Colloidal
structures and effects on pollution and biogeochemistry are an area of immense
importance but perhaps less relevant to this chapter. With the development and
increasing use of manufactured nanoparticles (NPs) (reviewed in Chapters 1 and
2), it is now necessary to consider how NPs and colloids interact and also treat the
aquatic colloid literature as a rich source of information on potential NP fate and
behaviour. Both NP properties (Chapters 2 and 3), their interaction with natural
colloids and the subsequent changes in NP properties will need to be understood
to understand NP transport and also (eco)toxicology (Chapters 7 and 9). In addi-
tion to the study of the physical interactions and fractionation of NPs with natural
colloids, those interested in NP fate and behaviour would be well served by review-
ing the now extensive literature on natural colloids which will facilitate develop-
ment in this novel fi eld.

4.9 References

Aiken , G. , D. McKnight , R. Harnish and R. Wershaw ( 1996 ) Geochemistry of aquatic humic
substances in the Lake Fryxell Basin, Antarctica : Biogeochem. , 34 , 157 - 88 .
Alasonati , E. , B. Stolpe , M. - A. Banincasa et al. ( 2007 ) Asymmetrical fl ow fi eld fl ow
fractionation - Multidetection system as a tool for studying metal- alginate interactions :
Environ. Chem. , 3 , 192 - 8 .