Environmental Engineering Reference
In-Depth Information
process involving (1) the deposition (accumulation) step, where metal ions are reduced at the chosen
E
d
for a fixed period of
time, followed by (2) the stripping step, where the accumulated metal is reoxidized through the application of a low constant
oxidizing current (stripping current,
I
s
). The analytical signal is the electrolysis time,
τ
(transition time), and therefore an SSCP
curve is a representation of
τ
over
E
d
, where the analytical signal
τ
always reflects the magnitude of the original deposition flux,
irrespective of its nature (i.e., diffusion controlled or kinetically controlled). The thermodynamic complex stability constant can
be calculated from the shift in the half-wave deposition potential, Δ
E
d,1/2
(analogous to the DeFord-Hume expression), irrespective
of the degree of lability of the system [55]:
*
RT
nF
τ
τ
(
)
+
′
∆
E
=−
ln
1
+
K
ln
ML
+
(32.5)
d
,/
12
*
M
*
and τ
*
are the limiting wave heights in the presence and absence of ligands, respectively,
n
is the number of
electrons involved in the faradaic process, and
K
′ is the conditional equilibrium parameter in conditions of excess ligand
(=
Kc
l,t
=
c
Ml
/
c
M
).
SSCP is especially advantageous when using a thin mercury film electrode (TMFE) since the rapid metal transport inside
the thin film during the stripping step implies that the measurements are always performed under conditions of complete
depletion. This allows the use of much higher stripping currents with the great advantage of the oxygen not having the time to
chemically oxidize the amalgamated metal [56]. The comparison of the stability constants obtained from the two SSCP signals
(the shift of the half-wave deposition potential and the decrease of transition time) provides information about the dynamic
nature of the metal complexes formed with the ligands present, in reasonable agreement with the predictions from the dynamic
theory for colloidal systems [55, 57].
The simultaneous use of AGNES and SSCP to study metal binding with heterogeneous ligands results in a great advantage
as they provide complementary data [6, 58].
where τ
ML
+
32.2.2.6 Bioavailability and Toxicity of NMs
Metal speciation is a crucial feature for the comprehension of the transport,
bioavailability, and toxicity of metal ions. FIAM and BlM, based on the concept that the formation of any complex in solution
will reduce trace metal uptake, successfully describe more than 90% of the metal ion bioavailability studied cases. Exceptions
to these models can be explained taking into account the dynamic aspects of interconversion of species and the possibility of
some inert complexes to be transported directly through the biological cell membrane (Fig. 32.2).
Bioavailability of metal-containing NMs differs from the free metal ion case in three aspects: (1) NMs may induce cell wall
and cell membrane damage; (2) NMs can be internalized; and (3) NMs will provide a point source of metal ions when dissolving.
Reaction layer
µ
Diffusion layer
Bulk solution
δ
D
M
M
z
+
Metal
ML
i
Organism/
electrode
k
d
k
a
Mass transport
M+L
i
k
i
nt
(M)
k
d
M+L
i
ML
i
Internalization
/ reduction
ML
i
Metal+Ligand
i
D
MLi
k
i
nt
(ML
i
)
fIgUre 32.2
Schematic representation of the concentration profiles for the metal (M) and the metal-ligand species (Ml
i
) as a function of
distance from the organism/electrode surface. The figure illustrates the biological, chemical, and physical parameters that govern the biouptake/
reduction of trace compounds:
k
int
, internalization rate constant;
k
a
, association rate constant of Ml
i
;
k
d
, dissociation rate constant of Ml
i
;
D
M
,
diffusion coefficient of M;
D
Ml
, diffusion coefficient of the Ml
i
species;
μ
, thickness of the reaction layer; and
δ
, thickness of the diffusion layer.
