Environmental Engineering Reference
In-Depth Information
for an ionic strength of 1.0 mM, but only an increase of 1.5× and 2.2× for an ionic strength of 5.0 mM, respectively, and barely
no increase within the experimental error for an ionic strength of 10 mM. Acrylamide gels are a good example of a modestly
charged gel with no significant intrinsic chemical binding. The same authors [43] also used alginate gels and found an increase
of free cadmium in the gel (3×) at an ionic strength of 10 mM, but, in contrast to acrylamide gels, the total level of free cadmium
was enhanced by a factor of approximately 60, resulting predominantly from the specific binding of the cadmium by the uronic
acids of the alginate gel. Alginate gels are not only more charged than acrylamide gels but also have a significant ability to
chemically bind cadmium, thus further enriching the gel phase in cadmium ions. These examples illustrate not only the tremen-
dous potential that the polymer layer may have to bind metal ions but also their variability depending on their respective charge
densities and the presence or absence of specific binding sites for the metal ions.
32.2.2.3 Self-Interaction
An interesting and often neglected aspect of the NM's fate and toxicity evaluation is that one of
the prime binding ligands for the dissolved metal ions from the NM is very probably the charged polymer in the stabilizing
layer. For a medium with low ionic strength and relatively high pH (higher than the p
K
a
of the acid groups of the shell), a rea-
sonably charged soft shell will have a major impact on the stability of the particle. In this situation, the free metal ion
concentration in the soft shell layer will be much larger than in the bulk, and, consequently, the NM metal core will be much
less likely to dissolve, since it will be in the vicinity of a much higher free metal concentration, closer to the saturation value.
This is a feature that is commonly used by manufacturers to effectively prevent their NMs from dissolving. Evidently, when
exposed to changes in the bulk, especially increases of ionic strength and lowest pH environments, the potential in the layer
might change significantly and the NMs will dissolve, becoming a source of metal ions to the bulk.
32.2.2.4 Stabilizer-Matrix Interactions
A stabilized NM is likely to suffer slower physical changes than a nonstabilized
NM, thus being available to interact with the components of the matrix. This leads to another often ignored aspect related to a
NM's ability to bind the metal ions previously present in the matrix, thus changing their speciation [6, 44]. The ability of the
core-shell NMs to interact with other matrix components, like humic and fulvic acid fractions [45], the rich variety of particulate
matter (among those several positively charged metal oxide particles), and living organisms (bacterial cell walls, algae mem-
branes, phytoplankton shells, etc.), is also well documented. On the face of it, it is important to remember that in toxicological
experiments, upon introducing the NM into the exposure media, the trace metal speciation and possibly the speciation of other
components of the matrix will change. Therefore, in order to understand the biouptake of toxic species, we first must understand
the nature of the changes yielded by NM-matrix interactions.
32.2.2.5 Speciation Techniques
The trace metal speciation techniques of interest can be separated in two groups: equilibrium
and dynamic techniques. Equilibrium techniques usually report the free metal ion concentration, while dynamic techniques
report the free metal ion plus the labile fraction of the complexed metal.
Although we will not describe it, we would like to mention the competitive ligand exchange adsorptive cathodic stripping
voltammetry (ClE-AdSV) [46]. The reason we do not a priori recommend this technique is the need to add a competitive ligand
to the solution, which is not desirable in many toxicological experiments.
32.2.2.5.1 Free Metal Ion Determination
Among the techniques that are able to determine the free metal ion concentration,
the most useful are the potentiometry with ion-selective electrodes (ISEs), the Donnan membrane technique (DMT), and the
absence of gradients and Nernstian equilibrium stripping technique (AGNES).
Potentiometry using ISEs is a well-established and known technique, and is probably the ideal method for free metal ion deter-
mination since the electrodes are easy to operate, and directly measure the free metal ion without disturbing the equilibrium or
the sample composition [47]. The drawback is that the ISEs' use is restricted to the quantification of proton and those elements
present in total concentrations above 10
−6
mol l
−1
due to the solubilization and contamination/adsorption of the electrode mem-
brane [48]. Although in the last decade the development of potentiometric sensors using polymeric membranes has increased the
ISEs' detection limit [49], their operation is still difficult and most are not commercially available. When the ISE is placed in a
solution containing the ion, a potential (
E
) is established between the membrane interface and the solution. This
E
is measured
against a reference electrode, and its magnitude is proportionally dependent on the free metal ion concentration in the bulk,
c
*
:
Ek
S
n
*
M
=+log
c
(32.2)
where
S
/
n
represents the slope (theoretically 2.303
RT
/
F
) and
k
is a constant dependent on electrode construction.
