Page 295 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

280 Principles and Methods

(fullerol aggregates, THF/nC , and TTA nC ) the initially hydroxy-
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60
lated fullerols appear to be relatively “immune” to changes in the solu-
tion chemistry, while the two varieties of nC 60 exhibited differential
changes in deposition within the column. The observation that fullerol
aggregates were relatively unaffected by the presence of the tannic acid
contrasts with the observation of reduced aggregation of fullerol in
tannic acid solutions (Figure 7.30). However, similar to the aggregation
experiments, tannic acid (TA), tended to stabilize the nC , resulting in
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less removal in the porous medium. Also, just as natural organic matter
in the form of polysaccharides tended to promote aggregation of nC 60
(Figure 7.28), algenate polysaccharides in solution also appear to
increase the fraction of nC 60 deposited in the porous medium, thereby
reducing fullerene mobility. Thus, adsorbing organic species in envi-
ronmental or physiological solutions may be expected to either increase
or decrease exposure and bioavailability, depending on the nature of the
organic matter.
Indeed, we have also observed that depending on the cell culture
media used, very different results are obtained regarding the propen-
sity of fullerenes to aggregate. For example, when placed in Dulbecco’s
Modified Eagle Medium (primarily a solution of glucose, amino and
other organic acids with inorganic salts with an ionic strength on the
1
order of 10 M) nC and fullerol colloids rapidly aggregate. In contrast,
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when placed in a solution of mesenchymal stem cell medium (MSC),
these materials are stabilized. Such differences in stability of nanoma-
terials in these two growth media underscore the importance of con-
sidering solute-nanoparticles interactions in performing toxicity studies
and in assessing environmental risks posed by fullerenes or other
nanomaterials.
Finally, we note that organic matter may also act to mobilize other-
wise immobile materials, as has been observed for organic acids in pro-
moting colloidal transport [90, 92]. This process of organic molecules
“piggybacking” on particles is referred to as facilitated transport, and
has also been observed for mercury and radionucleotides transported by
colloids [92].

Nanoparticle transport in matrices such
as gels and biofilms
Nanoparticles may be present in complex structures such as porous flocs
and biofilms. For example, as a result of heteroaggregation, highly random
structures may be formed that include nanoparticles within the structure.
In many environmental systems, surfaces exposed to nonsterile and
humid conditions can be colonized by microorganisms that encage them-
selves in hydrated exopolymeric substances (EPSs), which contain