Chemistry Reference
In-Depth Information
4
2
2
0
-2
-4
-6
-8
0.05
0.1
0.15
0.2
0.25
σ
p
FIGURE 1.7
Lethal and sublethal effects of metal ions on
Daphnia magna
are predictable
with the softness index (σ
p
). White diamonds = reproductive impairment with chronic exposure
(Log EC
50
after 3 weeks), grey squares = mortality with chronic exposure (Log LC
50
after
3 weeks), and black triangles = mortality with acute exposure (Log LC
50
after 48 hours).
Effects data were taken from Khangarot and Ray (1989) and Biesinger and Christensen (1972).
The regression
r
2
values for the linear models fit through the acute and chronic lethality, and
chronic reproductive impairment data were 0.83, 0.84, and 0.78, respectively.
Complementing this work, Weltje (2002) took the same approach to study the tri-
valent, class (a) lanthanide ions. This study was especially interesting for two reasons.
The rapidly growing, worldwide use of the rare earth elements (Haxel et al. 2002)
makes understanding and predicting lanthanide ion toxicities important (Zhang
et al. 1999). But more germane here, how electrons are added to the 4
f
shell of this
series of elements permits clear demonstration of how electrostatic interactions influ-
ence class (a) metal ion toxicity. The electron occupancy of the seven 4
f
orbitals
increases from 0 for lanthanum to 14 for lutetium (see Table 1 of Moeller [1975]
for exact configurations). Because the 4
f
electrons are not effective at shielding the
nuclear charge, a progressive decrease of ionic radii manifests from lanthanum to
lutetium, i.e., the lanthanide contraction (Barrett 2002). Consequently, the charge
density (reflected by
Z
2
r
−1
) for these trivalent ions increases steadily from lanthanum
to lutetium. Weltje (2002) demonstrated a corresponding increase in toxicity: biolu-
minescence inhibition increased as the energy of the metal ion increased relative to
electrostatic interactions with biochemical ligands.
Expanding the biological scale of organization still further outward to the whole
metazoan level, it is easy to find published relationships between metal ion qualities
and adverse effects. Newman et al. (1998) tabulated a series of such studies. Included
effects were acute and chronic lethality, and developmental and reproductive effects.
Test species ranged widely from
Planaria
to mice. Walker et al. (2003) also exam-
ined the literature addressing this theme, clarifying several important themes. And
more such relationships are published each year. As examples, Lewis et al. (1999)
produced QSAR models for mice and rats exposed to more than twenty metal ions
and Van Kolck et al. (2008) produced models for bioconcentration and lethality for
two mollusks.
