Chemistry Reference
In-Depth Information
replaced electrophilicity (ω) in the QSARs (
Table 5.20
). However, only the electro-
philicity (ω)-based QSAR was able to distinguish between As
3+
and As
5+
toxicity
Sacan et al.
(2009) developed 38 QSARs to predict the toxicity of cations to nitri-
fying bacteria (
Table 5.4
). They measured oxygen consumption and carbon dioxide
production from nitrifying bacteria in the presence of unspeciated cation concen-
trations and speciated (labile) cation concentrations. Labile concentrations included
the free metal concentrations plus the concentration of the metal-anion complexes
having weak stability constants (log K). A 50% reduction in oxygen consumption
and carbon dioxide production was calculated for each metal and the correspond-
ing IC
50
values were converted to logarithmic toxicity units as pTO
2
a nd pT CO
2
(Table 5.4). pTO
2
a nd pT CO
2
QSARs were developed for 8 unspeciated cations using
8 physicochemical properties (Table 5.20). The pTO
2
QSARs for 8 unspeciated cat-
ions with the highest
r
2
values, lowest SE values, and highest F values were those
developed with the cationic charge (Z) and the chemical potential of the gaseous
phase (µ
(g)
) (Table 5.20). The pTCO
2
QSARs for 8 unspeciated cations with the high-
est
r
2
values, lowest SE values, and highest F values were those developed with Z,
gaseous phase energy of HOMO energy (
E
HOMO(g)
), and the chemical potential of
the gaseous phase (µ
(g)
) (Table 5.20). Since the labile toxicity of Hg could not be
calculated, it was not included in the 7 cations used to develop pTO
2
-labile and
pT CO
2
-labile QSARs (Table 5.20). The pTO
2
-labile QSARs with the highest
r
2
val-
ues, lowest SE values, and highest F values were those developed with the chemical
potential of the aqueous phase (µ
(aq)
), µ
(g)
and the energy of the polarized solute sol-
vent (
E
PSS
) (Table 5.20). As a result of eliminating Ag
+3
and Cr
+3
(the most and least
toxic cations to nitrifiers), the pTO
2
-labile QSAR for only 5 divalent cations had a
higher
r
2
value, lower SE value, and higher F value than those QSARs developed for
7 cations (Table 5.20). The pTCO
2
-labile QSARs for 7 cations and 5 cations with the
highest
r
2
values, lowest SE values, and highest F values were those developed with
the
E
PSS
(Table 5.20). Using 2 physicochemical properties to develop QSARs for 8
unspeciated cations almost always consistently produced higher
r
2
value, lower SE
value, and higher F value than those QSARs that used 1 physicochemical property
(Table 5.20).
As noted above, Mendes et al.
(2010) developed QSARs for predicting cation tox-
icity using standard reduction-oxidation potential, electronegativity, and the Pearson
and Mawby (1967) softness parameter (
Table 5.12
)
. However, Mendes et al. (2010)
developed 9 QSARs for predicting cation toxicity using some less numerous physi-
cochemical properties (Table 5.20). The QSARs developed with the covalent index
X
r
m
2
)
(
2
and the absolute value of the logarithm of the first hydrolysis con-
stant (|log
K
OH
|) had the highest
r
2
values, lowest p values, and lowest AIC values
(Table 5.20).
Su et al.
(2010) developed 2 QSARs to predict the combined toxicity of phenols
and Pb to the bioluminescent bacterium
Vibrio fischeri
(formerly
Photobacterium
phosphoreum
)
(Table 5.4). However, since the 2 QSARs were not developed to pre-
dict cation toxicity, they were not included in Table 5.20.
Among the 128 QSARs in Table 5.20 that used less numerous physicochemi-
cal properties used to predict cation toxicity, there were some physicochemical
properties that were used more than a few times by different teams of investigators
or
X
r
m
